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!standard 03.04 (02)          03-09-02 AI95-00251/08
!standard 03.09.01 (02)
!class amendment 00-12-04
!status work item 00-12-04
!status received 00-12-04
!priority High
!difficulty Hard
!subject Abstract Interfaces to provide multiple inheritance
!summary
This proposal adds "interface" types to the standard as a kind of componentless abstract tagged type. A tagged type may be derived from one or more such interfaces; this provides a limited form of multiple inheritance. Dispatching calls through the primitives of the interface type dispatch to code bodies associated with specific tagged types that are derived from the interface.
!problem
A number of recent language designs have adopted a compromise between full multiple inheritance and strict single inheritance, using a concept called "interface" types. An interface consists solely of a set of operation specs -- the interface type has no data components and no operation implementations.
A type may "implement" multiple interfaces, but inherit code from only one parent type. This compromise has been found to have much of the power of multiple inheritance, without most of the implementation and semantic difficulties. This allows a single type to "masquerade" as a number of different types so that it may take advantage of existing abstractions that expect an object that provides a certain set of primitives.
This kind of flexibility is possible in Ada currently only using generics, or relatively complicated mechanisms using access discriminants. The solutions using generics don't really work when the abstraction itself cannot be represented as a generic. For example, a graphical subsystem may have a linked list of "observer" objects, each of which must implement an operation to receive notification when the observed object changes. This is not amenable to a generic mix-in approach. You can't mix-in the ability to be on a linked list of observers. Access discriminants can accomplish this, at the expense of making the type limited, and requiring a fair amount of mechanism. Finally, interfacing with Java and the new ".net" infrastructure from Microsoft would be significantly smoother if this concept were supported in a natural way in Ada.
!proposal
An interface type is a new kind of componentless abstract tagged type.
In addition to normal linear (i.e. single parent) derivation, a tagged type may now be derived from zero or more interface types, thereby inheriting their primitive subprograms.
The primitive subprograms of an interface type must either be abstract or "null" (See AI-348 for the definition of null procedures).
The class-wide type corresponding to an interface type covers all of the types derived from that interface type. This allows conversion between the classwide type and any type which is derived from the interface type.
In previous discussions (and in the discussion of the problem, above), there have been references to a specific type "implementing" an interface type. In this sense, a specific type can be thought of as "implementing" an interface if it is derived from that interface. This terminology ("implementing an interface") is not used in the RM wording, but it is useful for purposes of discussion.
A private extension may be derived from an interface type as well; the complete view and the partial view of a type must agree with respect to the set of interfaces they "implement".
Formal interface types are added to the language.
See the Wording section for details.
!discussion
All primitives of an interface are required to be abstract or null to minimize difficulty in resolving what happens when the "same" primitive is inherited from two interfaces, or from an interface and the parent type. If only the parent type can have a non-abstract, non-null primitive, no question arises as to which implementation to use. Languages that support full multiple inheritance need elaborate rules for resolving these kinds of conflicts.
No data components are allowed in interfaces to avoid implementation complexity and inefficiency. If data components could be inherited from multiple interfaces, or from an interface and the parent type, then the offset of a component of an interface could not be determined statically. It would make component access as complicated and expensive as calling a primitive (which is discussed below), or require recompilation of inherited primitives even when not overridden. Inheritance then becomes more like macro expansion.
The rules are structured to ensure that an existing componentless abstract tagged type can be changed to being an interface type with minimal disruption to the existing code. Because an interface type is an abstract tagged type, it can be used anywhere an abstract tagged private (or null record) type is permitted.
We disallow "private" inheriting from interfaces, because without allocating a complete separate set of dispatching operation "slots" for the corresponding operations, there is no good way to prevent these operations from being unintentionally overridden outside of the package by a descendant that explicitly implements the same interface. For example:
package P is type T1 is tagged private; ... private type T1 is new Foo and I with ... procedure Prim_Of_I(X : T1); end P;
... type T2 is new P1.T1 and I with ... procedure Prim_Of_I(X : T2); -- if operations of I are overridden for T2 (like Prim_Of_I), they -- will override operations associated with T1's "private" -- inheritance of I, violating the "privateness" of the -- inheritance.
In the above case, the partial view of T1 could be:
type T1 is new I with private;
This would make it visible that I is inherited, and thereby marking the primitives of "I" as fair game for overriding outside P.
Possible Implementation Model
As an implementation model, one can think of a type which inherits operations from an interface type as defining a mapping from the primitive operations of the interface type (perhaps represented as dispatch table slot numbers) to those of the inheriting type. A dispatching call to an operation of an interface type is then accomplished by mapping the called operation to the corresponding operation of the controlling operand's type. Alternatively, one might construct a separate dispatch table representing this alternative "view" of the "real" dispatch table, but that would require more work when deriving from a type which implements an interface. The slot-to-slot mapping remains valid and can be reused for the derived type, whereas a new alternative "view" of the dispatch table of the derived type would need to be constructed. A value of type Some_InterFace_Type'Class can then be represented at runtime as a record address paired (perhaps only conceptually - see next paragraph) with a reference to this dispatch table permutation map.
Consider the example of a call to Some_Interface_Type'Class'Input occuring as the controlling operand of a dispatching call. The implementation must be able to determine where to dispatch to in this case. In terms of the "slot-to-slot mapping" implementation model, this means that an implementation needs to provide a mapping at runtime which takes the tag of a specific tagged type and of an interface type (which is implemented by the specific type) and returns a reference to the associated dispatch table permutation map (aka the "slot-to-slot" map). Given this mapping, a "thin pointer" representation becomes feasible. In other words, a value of type Some_InterFace_Type'Class could be represented as an unaugmented record record address.
Alternatively, a "fat pointer" implementation model is possible. An interface "view" of an object is represented as a pair of pointers, one to an interface-specific dispatch table (or permutation map), and the other to the tagged object of the implementing type. The interface-specific dispatch table would have a layout determined by the order in which the primitives of the interface were declared, while its content would be determined by the particular type implementing the interface.
Intermediate approaches between these two extremes are also possible (e.g. use the thin pointer representation, but associate an implicit parameter with formal parameters). Note also that the thin/fat implementation decision is orthogonal to the decision about whether to store the details of how a type implements an interface (hereafter referred to as an "interface implementation descriptor") as a new dispatch table, as a slot-to-slot map, or in some other form.
In any case, a dispatch table could contain a map of some kind (e.g. an array) mapping interface type tag values to these "interface implementation descriptors", with one entry in the map for each interface implemented by the type. This would meet the aforementioned need for a mapping from the tag of a specific type and the tag of an interface type which it implements to the corresponding interface implementation descriptor.
Contrary to earlier discussions, an interface-to-interface view conversion requires no special support. The tag of the source object is that of a spcific type and so what is really being implemented is a specific-to-interface view conversion, the only added wrinkle being that the specific type is not known statically.
----
Random notes:
1) Inherited homographs must be overridden. If the homographs are
not mode-conformant, then this may be impossible. Life's hard.
2) Nothing here (in particular, nothing in the 8.3 stuff) is intended to
change the semantics of a program which declares no interface types.
3) An example to illustrate some of the 8.3 stuff:
package P1 is type T1 is interface; procedure P (X : T1) is abstract; -- P'1 end P1;
package P2 is type T2 is interface with P1.T1; -- P'2 (implicit) procedure P (X : T2) is abstract; -- P'3 end P2;
type D is new Some_Tagged_Type and P1.T1 and P2.T2 with null record;
D inherits procedures P'4, P'5, and P'6, corresponding to P'1, P'2, and P'3, respectively.
Since P'3 overides P'2, P'6 overrides P'5 by the "overriding is preserved by inheritance" rule.
Since P'2 corresponds to P'1, P'5 overrides P'4 by the "inherited copies of your parent's ops hide inherited copies of your grandparent's ops" rule.
4) Yes, it really is illegal if the parent type of a private extension's
completion happens to be descended from some interface that the private view was not descended from. This may turn out to be a pain.
This rule is, unfortunately, necessary. If this example,
package P is packge Pkg is type Ifc is interface; procedure Foo (X : Ifc) is abstract; end;
type Parent_1 is tagged null record;
type T1 is new Parent_1 with private; private type Parent_2 is new Parent_1 and Pkg.Ifc with null record; procedure Foo (X : Parent_2); -- Foo #1
type T1 is new Parent_2 with null record; end;
with P; package P_Client is type T2 is new P.T1 and P.Pkg.Ifc with null record; procedure Foo (X : T2); -- Foo #2 X : T2; end P2_Client;
with P_Client; package body P is ... begin Pkg.Foo (Pkg.Ifc'Class (P_Client.X)); -- call Foo #2 Pkg.Foo (Pkg.Ifc'Class (T1 (P_Client.X))); -- call Foo #1 end P2;
, were legal (it is illegal because the completion of T1 is descended an interface that the partial view is not descended from), then we would have two dispatching calls to Pkg.Foo with the two controlling operands having the same tag and yet different bodies would be executed. The two conversions to Pkg.Ifc'Class would map Pkg.Foo to different slots in the same dispatch table because the source types of the conversions are different.
We want to prevent this sort of thing.
5) A type derived from an interface type should inherit the same subprograms
as a type which is derived from both the interface type and an ancestor of the interface type. The two type definitions should be indistinguishable, in the sense that adding or deleting the explicit redundant derivation relationship should be a semantics-preserving transformation.
6) It is intended that there should be no real difference between the two
forms of derivation from interface types. For example, replacing the declaration
type T is new Interface_Type_1 and Interface_Type_2 with null record;
with
type T is new Interface_Type_2 and Interface_Type_1 with null record;
should be semantics-preserving.
7) Someone asked why an interface type declared in a visible part is not
allowed to have primitive operations declared in the private part. A dispatching call to a primitive of an interface type will execute the body of a corresponding routine associated with the specific type of the controlling operand. Without this restriction, it is possible that the specific type might provide no such routine. It would be ok to follow the example of 3.9.3(10) and allow this in the case where the subprogram declared in the private part "is overriding an abstract subprogram implicitly declared in the visible part", but this doesn't seem to be worth the bother because this could only be used to override an abstract procedure with a null procedure. There does not appear to be universal agreement on this point.
8) Someone asked why a limited interface cannot be implemented by a
non-limited type. An implementation would no longer know statically whether a dispatching function call is a call to a function with a return-by-reference result. This could be implemented, but it seems like an unnecessary complication. The language has, as a matter of general principle, steered away from constructs which would allow a limited view of an object of a non-limited tagged type.
9) It is intended that in the case where the implementation
arbitrarily chooses one overrider from among a group of inherited subprograms, users should be unable to determine which member was chosen. The "arbitrarily choose an overrider" monkey business is needed in order to allow
package Outer is package P1 is type Ifc1 is interface; procedure Null_Procedure (X : Ifc1) is null; procedure Abstract_Subp (X : Ifc1) is abstract; end P1;
package P2 is type Ifc2 is interface; procedure Null_Procedure (X : Ifc2) is null; procedure Abstract_Subp (X : Ifc2) is abstract; end P2;
type T is new P1.Ifc1 and P2.Ifc2; end Outer;
without requiring that T explicitly override any of its inherited operations. If requiring explicit overrides in this case is considered to be ok, then that bit of oddness could be deleted from 8.3.
If it turns out that users can somehow determine which member was chosen and that this approach introduces serious portability problems, then one could impose an arbitrary-but-determininistic tie-breaking scheme (e.g. make use of the order of the interface types, prefering operators inherited from "earlier" types; in the above example, this would mean that the operators inherited from P1.Ifc1 would override those inherited from P2.Ifc2).
!example
An example involving interface types:
type T is tagged null record; T_Var : T;
package P1 is type Ifc1 is interface; procedure Op (X : Ifc1) is abstract; procedure Op1_A (X : Ifc1) is abstract; procedure Op1_B (X : Ifc1) is abstract;
type Ref is access all Ifc1'Class; procedure Foo (X : Ref); end;
package P2 is type Ifc2 is interface; procedure Op (X : Ifc2) is abstract; procedure Op2_A (X : Ifc2) is abstract; procedure Op2_B (X : Ifc2) is abstract;
type Ref is access all Ifc2'Class; procedure Foo (X : Ref); end;
package body P1 is ...
procedure Foo (X : Ref) is type Stream_Ptr is access all Ada.Streams.Root_Stream_Type'Class; The_Stream : constant Stream_Ptr := ...; begin Op1_A (X.all); Op1_B (X.all); Ifc1'Class'Output (The_Stream, X.all);
if X.all in T'Class then T_Var := T (X.all); end if; end Foo; end P1;
package body P2 is ... end P2;
type D is tagged with P1.Ifc1 and P2.Ifc2 and record F1, F2 : Integer; end record;
procedure Op1_A (X : D); procedure Op1_B (X : D); procedure Op (X : D); procedure Op2_A (X : D); procedure Op2_B (X : D);
...
X : aliased D;
begin P1.Foo (X'access); P2.Foo (X'access); P1.Op (P1.Ifc1'Class (X)); P2.Op (P2.Ifc2'Class (X)); end;
--------
A somewhat less artificial example:
package Object_Monitoring is type Monitored_Object is interface; procedure Display (Object : Monitored_Object) is abstract;
type Object_Reference is access all Monitored_Object'Class; for Object_Reference'Storage_Size use 0;
procedure Register (Object : Object_Reference); procedure Unregister (Object : Object_Reference); procedure Display_Registered_Objects; end;
package body Object_Monitoring is package Object_Reference_Sets is new ... ;
Registered_Objects : Object_Reference_Sets.Set;
procedure Display_One (Object : Object_Reference) is begin Display (Object.all); end;
procedure Display_Registered_Objects is begin Object_Reference_Sets.Visit_All (The_Set => Registered_Objects, Visit_One => Display_One'access); end;
... end Object_Monitoring;
package Pkg1 is ... end Pkg1;
with Object_Monitoring; package body Pkg1 is type T is tagged record F1, F2 : Integer; end record;
type Monitored_T is new T and Object_Monitoring.Monitored_Object with ...;
procedure Display (Object : Monitored_T) is begin ... end;
X : aliased Monitored_T;
... begin Object_Monitoring.Register (X'access); end;
!wording
3.2.1 Type Declarations
Add to syntax
type_definition ::= ... | interface_type_definition
3.4 Derived Types and Classes
Replace syntax section with
interface_list ::= interface_subtype_mark {and interface_subtype_mark}
derived_type_definition ::= [abstract] new parent_subtype_indication [[and interface_list] record_extension_part]
Add at the end of paragraph 3:
A derived type has one parent type and zero or more interface ancestor types.
Replace paragraph 8 with:
Each class of types that includes the parent type or an interface ancestor type also includes the derived type.
Add after paragraph 25:
If a type declaration names an interface type in an interface list, then the declared type inherits any user-defined primitive subprograms of the interface type in the same way.
Add after paragraph 35 (in the Notes section):
18 An interface type which has an interface ancestor "is derived from" that type, but it is not "a derived type". Conversely, a derived type is never an interface type.
3.4.1 Derivation Classes
Insert after the first sentence of paragraph 2:
A derived type or interface type is also derived from each of its interface ancestor types, if any.
Replace the last sentence of paragraph 10 with:
The ultimate ancestor of a type is the ancestor of that type, if any, that is not a descendant of any other type and that is not an interface type (see 3.9.4).
3.9.3 Abstract Types and Subprograms
Replace paragraphs 4-5 with
If a type inherits a subprogram corresponding to an abstract subprogram or to a function with a controlling result, then
- If the inheriting type is abstract or untagged, the inherited subprogram is abstract.
3.9.4 Interface Types
This section is entirely new.
An interface type is an abstract tagged type intended for use in providing a restricted form of multiple inheritance. A tagged type may be derived from multiple interface types.
Syntax
interface_type_definition ::= [limited] interface [with interface_list]
Legality Rules
An interface type (also called an "interface") is a specific abstract tagged type that is defined by an interface_type_definition.
An interface type shall have no components.
All user-defined primitive subprograms of an interface type shall be abstract subprograms or null procedures.
The type of a subtype named in an interface_list shall be an interface type.
If a type declaration names an interface type in an interface list, then the accessibility level of the declared type shall not be statically deeper than that of the interface type; also, the declared type shall not be declared in a generic body if the interface type is declared outside that body.
A descendant of an interface type shall be limited if and only if the interface type is limited.
A full view shall be a descendant of an interface type if and only if the corresponding partial view (if any) is also a descendant of the interface type.
For an interface type declared in a visible part, a primitive subprogram shall not be declared in the private part.
In addition to the places where Legality Rules normally apply (see 12.3), these rules apply also in the private part of an instance of a generic unit.
4.5.2 Relational Operators and Membership Tests
Replace paragraph 3 with
The tested type of a membership test is the type of the range or the type determined by the subtype_mark. If the tested type is tagged, then the simple_expression shall resolve to be of a type that is convertible (see 4.6) to the tested type; if untagged, the expected type for the simple_expression is the tested type.
4.6 Type Conversions
Replace paragraphs 21-23 with
If the target type is tagged then either:
- The operand type shall be covered by or descended from the target type; or
- The operand type shall be a class-wide type that covers the target type; or
- The operand and target types shall both be class-wide types and the specific type associated with at least one of them shall be an interface type.
If the target type is not included in any of the above five cases, there shall be a type that is an ancestor of both the target type and the operand type.
6.1 Subprogram Declarations
Replace
subprogram_specification ::= procedure defining_program_unit_name parameter_profile
with
procedure_specification ::= procedure defining_program_unit_name parameter_profile subprogram_specification ::= procedure_specification
7.3 Private Types and Private Extensions
Replace
private_extension_declaration ::= type defining_identifier [discriminant_part] is [abstract] new ancestor_subtype_indication with private;
with
private_extension_declaration ::= type defining_identifier [discriminant_part] is [abstract] new ancestor_subtype_indication with [interface_list and] private;
8.3 Visibility
Add after paragraph 12:
An implicit declaration of an inherited subprogram which is inherited from a non-interface type overrides that of a subprogram inherited from an interface type which is implicitly declared at the same point.
If one subprogram overrides another and a type inherits subprograms corresponding to both which are implicitly declared at the same point, then the inherited subprogram corresponding to the overriding subprogram overrides the other inherited subprogram.
If an inherited subprogram corresponds to another subprogram and a type inherits subprograms corresponding to both which are implicitly declared at the same point, then the subprogram corresponding to the inherited subprogram overrides the other inherited subprogram.
If a descendant of an interface type inherits two or more fully conformant abstract or null subprograms which are implicitly declared at the same point, and if the preceding rules do not cause any of them to be overridden, then one of these subprograms overrides the others; which one is unspecified, except that an abstract subprogram shall not be chosen if a null subprogram is available.
Add after paragraph 26:
If a descendant of an interface type inherits two homographs which are implicitly declared at the same point, then at least one shall be overridden.
12.5 Formal Types
Add to syntax
formal_type_declaration ::= ... | formal_interface_type_definition
12.5.1 Formal Private Types
Replace
formal_derived_type_definition ::= [abstract] new subtype_mark [with private]
with
formal_derived_type_definition ::= [abstract] new subtype_mark [with [interface_list and] private]
Add after paragraph 15:
The actual type shall be a descendant of every ancestor of the formal type.
12.5.5 Formal Interface Types
This section is entirely new.
The class determined for a formal interface type is the class of all interface types.
Syntax
formal_interface_type_definition ::= interface_type_definition
Legality Rules
The actual type shall be a descendant of every ancestor of the formal type.
The actual type shall be limited if and only if the formal type is limited.
13.14 Freezing Rules
Add after paragraph 7:
The declaration of a specific descendant of an interface type freezes the interface type.
!ACATS test
!appendix

!from Tucker Taft
!date Saturday, November 25, 2000 1:51 PM

Gary Dismukes wrote:
>
> > Here is a first cut on the Multiple (interface) Inheritance AI.
> > Comments welcome!
> > -Tuck
> (0-line AI included :-)
>
> Tuck, looks okay for a first cut but could use a little more detail ;-)
>
> -- Gary

Oops.  Here it is for real... (now that I have built up the suspense ;-)
-Tuck
----------
!standard 03.04    (02)                               00-11-21  AI95-xxx/01
!standard 03.09.01 (02)
!class amendment 00-11-21
!priority High
!difficulty Hard
!subject Tagged Types, Abstract Interface, Multiple Inheritance

!summary

This amendment AI proposes that "abstract interface" types may be
defined, and that a tagged type may "implement" one or more
such abstract interfaces.  The classwide type associated with
the abstract interface "covers" all types that implement it.
Dispatching calls through the primitives of the abstract interface
type dispatch to code bodies associated with specific tagged types that
implement the interface.

!question


!recommendation


Here are the proposed syntactic changes to support abstract interfaces:

   type_definition ::= ... | abstract_interface_definition

   abstract_interface_definition ::= ABSTRACT [LIMITED]

   derived_type_definition ::=
     [ABSTRACT] NEW parent_subtype_indication
       [AND abstract_interface_list]
       [record_extension_part]

   abstract_interface_list ::=
     absract_interface_subtype_mark {AND abstract_interface_subtype_mark}

   private_extension_declaration ::=
     ... [ABSTRACT] NEW ancestor_subtype_indication
       [AND abstract_interface_list] WITH PRIVATE;

   formal_type_definition ::= ... | formal_abstract_interface_definition

   formal_abstract_interface_definition ::=
       abstract_interface_definition

   formal_derived_type_definition ::=
     [ABSTRACT] NEW ancestor_subtype_indication
       [AND abstract_interface_list] [WITH PRIVATE];

An abstract interface type (or "abstract interface" for short)
is defined by an abstract_interface_definition or by a
derived_type_definition where the word ABSTRACT appears, the
parent type is an abstract interface, and there is no
record_extension_part.
All primitive operations of an abstract interface type must be declared
abstract (or perhaps "is null"?).  Only abstract interface types
may be mentioned in an abstract_interface_list.

A tagged type may declare that it "implements" an abstract
interface by mentioning it after the reserved word "NEW" or "AND".
Note that a tagged type defined by a derived_type_definition must
include a
record_extension_part.

If an abstract interface mentions other abstract interface types
in its definition (after the reserved word NEW or AND), then any
type that implements this new interface must also implement these
other abstract interfaces.  The new abstract interface is also said to
"implement" the other interfaces that it mentions.  An abstract
interface "implements" itself as well.  Finally, a tagged type
"implements" all of its ancestor tagged types, including itself.

Note that we allow an abstract interface type
as the parent type of a record extension or a private extension
to simplify the syntax, and to allow an abstract interface type and
an abstract tagged type to be used in a very similar fashion.
This allows one to switch from an abstract tagged type to an
abstract interface type during maintenance without significant
disruption.  [One possible goal would be for us to change
Ada.Finalization.[Limited_]Controlled into abstract interface
types.  This might require us to define an alternative to "is abstract'
such as "procedure ... is null" whereby an abstract interface
could establish a "null" default for an operation, since that is
what Controlled provides fopr all of its operations.]

If the reserved word LIMITED appears in an abstract_interface_definition,
the abstract interface is an abstract limited interface, and assignment
and equality are not available on its classwide type; otherwise
it is an abstract nonlimited interface.  A nonlimited type may implement
an abstract limited interface, but not vice-versa.  A derived type is
limited if and only if its parent type is limited.

A type that implements (directly or indirectly) an abstract interface inherits
the interface's (abstract) primitive operations with the usual substitution
of the new type for the abstract interface type in their profile.
If a type inherits multiple primitive subprograms that are
homographs of one another, they must be subtype conformant with one
another.  Also, a non-abstract inherited subprogram overrides any
abstract ones, and if they are all abstract, they must be fully conformant
with one another or be overridden [so the formal parameter names and
defaults are well-defined].  If a type is non-abstract,
and it inherits any abstract primitives that are not overridden
by inherited non-abstract ones, it also must (as usual) override them.

The 'Class attribute is defined for abstract interface types.
The classwide type associated with an abstract interface "covers"
all types that implement it, and their associated classwide types.
Conversions are permitted between (the classwide type of) any
descendant of a type that implements an abstract interface and
the classwide type of the abstract interface.  Converting to a
covered type generally requires a tag check.

Membership tests are permitted where the operand's type covers the
tested type.

Note that only dispatching calls are permitted on the primitives of
an abstract interface, since they are all abstract.

A tagged type matches a generic formal private extension so long as it
implements all the types mentioned in the formal_derived_type_definition.
Similarly, if the record extension that completes a private extension
declaration must implement all the types mentioned
in the private extension declaration.

An abstract interface matches a formal derived type without the words
"WITH PRIVATE" so long as the word ABSTRACT appears and
it implements all the abstract interfaces mentioned in the
formal_derived_type_definition.

        Implementation Model

A possible implementation model for an abstract interface reference
is to use a pair of pointers, one to an interface-specific dispatch
table,
and the other to the tagged object of the implementing type.
The interface-specific dispatch table would have a layout determined
by the order in which the primitives of the abstract interface
were declared, while its content would be determined by the
particular type implementing the interface.

If a given type (including potentially an abstract interface) implements one
or more abstract interfaces, appropriate dispatching tables for these
other interfaces must be pointed-to from the given type's dispatch
table, at
known offsets.  This allows a conversion from the given type to
one of these interfaces to be performed efficiently, by
fetching the desired interface-specific pointer from the appropriate
slot
in the given type's dispatch table.

Converting from an abstract interface to a non-interface
type that implements it can be performed using the same approach used
in Ada 95 to convert to a descendant tagged type, by checking a table
of ancestors to see if the target type appears at the appropriate level
in the table.  On the other hand, converting from one abstract interface
to another one that implements it will generally require more overhead,
both for the conversion and the associated run-time check (essentially
the same logic would be involved in a membership test).

One mechanism to support interfact-to-interface conversion is for every
tagged type to have in its dispatch table a pointer to an array of all
the interfaces it implements, as well as a pointer to a parallel array
with
the corresponding interface-specific dispatch tables.  (This pair of
parallel arrays could of course be combined into a single array
of pairs.)  To convert from one interface to another, one must
be sure that the type implements the target interface.
This can be done by scanning down this array.  If the target
interface is found, then the conversion would pick up the corresponding
interface-specific dispatch table from the parallel array to
form the two-word interface reference.

Note that this same array of interface-specific dispatch tables can
be used to support the conversion from a tagged classwide type
to an interface it is known to implement, by ensuring the array is
in the same order in all descendants of a given tagged type, and
it is only added to on the end as more interfaces are implemented
by lower-down descendants.  This conversion would not require
any searching, since the relevant offsets would be known
at compile-time by suitable ordering of the arrays.

****************************************************************

From: Tucker Taft
Sent: Monday, December 04, 2000 9:44 AM
Subject: Multiple inheritance bug bites ESA

For whatever reason, it sounds like ESA now believes multiple
inheritance is essential in some situations.  Groan.
Perhaps the multiple inheritance AI needs a slightly higher profile ;-)
-Tuck
--------------------
From team-ada@ACM.ORG Mon Dec  4 08:33 EST 2000
MIME-Version: 1.0
Date:         Mon, 4 Dec 2000 15:31:46 +0200
From: Soeren Henssel <Soeren.Henssel-Rasmussen@NOKIA.COM>
Subject:      ESA now also "prefer" C++ instead of Ada on some projects

European Space Agency has designed a prototype for component-based software
framework for a satellite Attitude and Orbit Control System (AOCS). Their
home page is at =>
http://www.softwareresearch.net/AocsFrameworkProject/ProjectHomePage.html
It has been programmed in C++ in preference to Ada 95 - read inter alia
http://www.softwareresearch.net/AocsFrameworkProject/DesignPrinciples.html
paragraph "Language Compatibility" for the reasons behind the decission. The
main reason is lack of multiple inheritance in Ada 95.

****************************************************************

From: Randy Brukardt
Sent: Monday, December 04, 2000 10:31 PM

While editing Tucker's proposal, I was struck by the following:

> Note that we allow an abstract interface type
> as the parent type of a record extension or a private extension
> to simplify the syntax, and to allow an abstract interface type and
> an abstract tagged type to be used in a very similar fashion.
> This allows one to switch from an abstract tagged type to an
> abstract interface type during maintenance without significant
> disruption.  [One possible goal would be for us to change
> Ada.Finalization.[Limited_]Controlled into abstract interface
> types.  This might require us to define an alternative to "is abstract'
> such as "procedure ... is null" whereby an abstract interface
> could establish a "null" default for an operation, since that is
> what Controlled provides fopr all of its operations.]

This "goal" seems to me to be a good way to kill off this proposal. The effect
of this would be to require a significant change in the way that finalization is
implemented. For instance, if the implementation literally uses a list of
Ada.Finalization.Controlled'Class to implement this, the pointer size would
change (assuming the use of the implementation model given in the AI).
Similarly, the implementation would need to handle any additional data needed
for finalization by some compiler magic, rather than by inheritance as is
currently done (assuming that abstract interface types may not have components).

I note that the proposal never seems to state that abstract interface types must
not define any components. Certainly, the implementation model and the entire
discussion seem to assume that is the case. (An "normal" abstract type can have
components defined, they'll be part of any extension; but I don't think we want
that here.)

****************************************************************

From: Tucker Taft
Sent: Wednesday, December 20, 2000 5:51 PM

Randy Brukardt wrote:

> While editing Tucker's proposal, I was struck by the following:
> ... [One possible goal would be for us to change
> > Ada.Finalization.[Limited_]Controlled into abstract interface
> > types.  This might require us to define an alternative to "is abstract'
> > such as "procedure ... is null" whereby an abstract interface
> > could establish a "null" default for an operation, since that is
> > what Controlled provides fopr all of its operations.]
>
> This "goal" seems to me to be a good way to kill off this proposal. The
> effect of this would be to require a significant change in the way that
> finalization is implemented. For instance, if the implementation literally
> uses a list of Ada.Finalization.Controlled'Class to implement this, the
> pointer size would change (assuming the use of the implementation model
> given in the AI). Similarly, the implementation would need to handle any
> additional data needed for finalization by some compiler magic, rather than
> by inheritance as is currently done (assuming that abstract interface types
> may not have components).

I realized this was a possibility, but I wasn't sure what
current implementation strategies actually are.  Comments
from vendors about specific implementation problems
with this possible change would certainly be of interest.

In any case, I certainly hope that the
decision whether to use abstract interfaces for
Ada.Finalization.Controlled can be viewed as a completely separate
issue.  I would suspect that there are other similar situations,
even if Finalization turns out not to work for other reasons, where
one would want to move from abstract tagged type to abstract
interface or vice-versa during maintenance or enhancement.
That is one of the main advantages of the
proposed syntax, and is why I changed the suggested syntax
from what I presented in Baltimore.

> I note that the proposal never seems to state that abstract interface types
> must not define any components. Certainly, the implementation model and the
> entire discussion seem to assume that is the case. (An "normal" abstract
> type can have components defined, they'll be part of any extension; but I
> don't think we want that here.)

I improperly presumed the reader knew what was meant by an
abstract "interface" type.  It definitely doesn't have any
data components.  That is one of the things that significantly simplifies
multiple inheritance of interfaces.

I also notice that an example is missing.  I will try to provide
one over the next couple of days.

****************************************************************

From: Steve Baird
Sent: Thursday, May 16, 2002  2:38 PM

This is an initial proposal for wording for AI-00251 and some
related issues (null procedures and unreserved keywords).
    -- Steve

----------------------------------------------------------------
----------------------------------------------------------------

Null procedures:

    3.1

    Add
        basic_declaration ::= ... | null_procedure_declaration

    --------

   6.1

   Replace
       subprogram_specification ::= PROCEDURE defining_program_unit_name
                                              parameter_profile
   with
       procedure_specification ::= PROCEDURE defining_program_unit_name
                                             parameter_profile
       subprogram_specification ::= procedure_specification

   --------

   6.7  Null Procedures

                                  Syntax

        null_procedure_declaration ::= procedure_specification IS NULL;

                                  Static Semantics

        A null_procedure_declaration declares a null procedure.

                                  Dynamic Semantics

        The execution of a null procedure is invoked by a subprogram call.
        This execution has no effect.

   --------

Discussion:
   1) Like an instantiation, a null procedure is not allowed as a
completion.
      Allowing this would double the amount of RM text needed.
   2) A null procedure may have OUT-mode parameters, the same as a
      conventional "begin null; end;" procedure.
   3) Change Ada.Finalization spec to declare Initialize/Finalize/Adjust
      as null procedures?

----------------------------------------------------------------
----------------------------------------------------------------

Unreserved keywords (see AI-284):

    2.2(1)
      Replace "a reserved word," with "a keyword,"

    2.3(4)
      Replace
         An identifier shall not be a reserved word.
      with
         An identifier shall not be a reserved keyword.

    Replace 2.9 with:

        2.9 Keywords

                                  Syntax
        The following are the reserved keywords (ignoring upper/lower case
        distinctions):
            ABORT ... XOR [same list as before]

        The following are the unreserved keywords:
            INTERFACE

    Throughout the RM:
        Replace "reserved word" with "keyword" (e.g. 3.9.3(2)).

----------------------------------------------------------------
----------------------------------------------------------------

Abstract Interface Types

    3.2.1

    Add
        type_definition ::= ... | abstract_interface_definition

    --------

    3.4(2)

    Replace
        derived_type_definition ::=
            [ABSTRACT] NEW parent_subtype_indication
            [record_extension_part]
    with
        derived_type_definition ::=
            [ABSTRACT] NEW parent_subtype_indication
            [[AND abstract_interface_list] record_extension_part]

    --------

    3.4

    Add after paragraph 23:
        A type which implements an abstract interface (see 3.9.4) inherits
        subprograms from the interface type, in the same way as for a type
        derived from the interface type.

    --------

    3.9.1

    Add between sentences 3 and 4 of paragraph 3:
         The accessibilty level (see 3.10.2) of a type which implements an
         abstract interface (see 3.9.4) shall not be statically deeper than
         that of the interface type.

    Add after sentence 1 of paragraph 4:
         An abstract interface shall not be
         implemented in a generic body if the interface type is declared
         outside that body.

    --------

    3.9.3

     Add after paragraph 6:

         Corresponding rules apply for a type which inherits an abstract
         subprogram by implementing an abstract interface (see 3.9.4, 3.4).

    --------

    3.9.4 Abstract Interface Types

        An abstract interface type is an abstract tagged type intended for use
        in providing a restricted form of multiple inheritance.
        A tagged type may "implement" multiple interfaces, thereby allowing
        multiple views of objects of the type.

                                 Syntax

        abstract_interface_definition ::= ABSTRACT [LIMITED] INTERFACE

        abstract_interface_list ::=
            abstract_interface_subtype_mark
            {AND abstract_interface_subtype_mark}

                                 Legality Rules

        An abstract interface type (also called an "interface type" or
        "interface") is a specific abstract tagged type that is defined by an
        abstract_interface_definition, or by a derived_type_definition or
        formal_derived_type_definition where the keyword ABSTRACT
        appears and the parent type is an interface type.

        An interface type shall have no components.

        All primitive operations of an interface type shall be abstract
        subprograms or null procedures.

        The type of a subtype named in an abstract_interface_list shall be
an
        interface type.

        A descendant of a type which names an interface type in an
        abstract_interface_list, or which is derived from an interface type,
        is said to "implement" the interface and all of its ancestors.
        A class-wide type implements the interfaces implemented by the
        corresponding specific type. The corresponding full view of a
        partial view of a type implements all interfaces implemented by
        the partial view.

        A type which implements an interface shall be limited if and only
        if the interface type is limited.

        If a partial view defines an interface type, then the corresponding
        full view shall define an interface type.

        If a full view implements an interface, then the corresponding
        partial view (if any) shall implement the interface.

        For an interface type declared in a visible part, a primitive
        subprogram shall not be declared in the private part.

        In addition to the places where Legality Rules normally apply
        (see 12.3), these rules apply also in the private part of an
instance
        of a generic unit.

    --------

    4.5.2

    In paragraph 3, replace:

        If the tested type is tagged, then the simple_expression shall
resolve
        to be of a type that covers or is covered by the tested type;

    with

        If the tested type is tagged, then the simple_expression shall
resolve
        to be of a type that covers or is covered by the tested type, or of
a
        class-wide type which covers an abstract interface type that is
        implemented by the tested type.

    --------

    4.6

    Add after paragraph 20:

        If the target type is tagged and there exists no type that is an
        ancestor of both the target type and the operand type, then either:

            The target type shall cover or be an ancestor of an abstract
            interface type which is implemented by the operand type; or

            The operand type shall be a class-wide type that covers an
abstract interface type which is
            implemented by the target type.


    In paragraph 21, replace "four" with "five".

    --------

    7.3

    Replace
        private_extension_declaration ::=
          type defining_identifier [discriminant_part] is
          [ABSTRACT] NEW ancestor_subtype_indication WITH PRIVATE;
    with
        private_extension_declaration ::=
          type defining_identifier [discriminant_part] is
          [ABSTRACT] NEW ancestor_subtype_indication
          [[AND abstract_interface_list] WITH PRIVATE];

    --------

    8.3

    Add after paragraph 12:

        An implicit declaration of a subprogram which is neither abstract
nor
        a null procedure overrides that of a subprogram simultaneously
        inherited by implementing an abstract interface.

        If one subprogram overrides another and a type simultaneously
inherits
        subprograms corresponding to both, then the inherited subprogram
        corresponding to the overriding subprogram overrides the other
        inherited subprogram.

        If an inherited subprogram corresponds to another subprogram and a
        type simultaneously inherits subprograms corresponding to both, then
        the subprogram corresponding to the inherited subprogram overrides
the
        other inherited subprogram.

    Add after paragraph 23:

        If two non-overridden homographs are simultaneously inherited as
        primitive operations of a tagged type which implements an abstract
        interface, then they shall be fully conformant and either both shall
        be abstract subprograms or both shall be null procedures.

    --------

    12.5.1

    Replace
        formal_derived_type_definition ::= [ABSTRACT] NEW subtype_mark
                                           [WITH PRIVATE]
    with
        formal_derived_type_definition ::= [ABSTRACT] NEW subtype_mark
                                           [[AND abstract_interface_list]
                                           WITH PRIVATE]


    Add after paragraph 18:

        If the formal type is an abstract interface type, then the actual
type
        shall be an abstract interface type.

        The actual type shall implement any interfaces that are implemented
by
        the formal type.

    --------

    13.14

    Add after paragraph 7:

        The implementation of an abstract interface causes freezing of the
        abstract interface type.

    --------

Discussion:

   1) The "no components for interface types" rule does not conflict with
the
      "every tagged object has a tag" rule.
      13.5.1(15) might be interpreted to mean that a tag is an
      "implementation-defined component" of a tagged type, but apparently an
      "implementation-defined component" is not a "component" (see, for
      example, the definition of "needed components" for record aggregates).

   2) An abstract interface type is an abstract tagged type; this point was
      unclear in the AI.
      The class-wide type associated with an interface type is not an
      interface type.

   3) It is intended that explicitly specifying that a type implements an
      interface which it already implements should have no semantic effect.

   4) A non-interface type may be derived from an interface type.

   5) An abstract non-interface type cannot be directly derived from an
      interface type. The syntax given in the AI for doing this
      would violate the rule that an extension part is given as part of
      a derivation if and only if the parent type is tagged.

      I feel that preserving this rule is important. I don't want to take a
      clean, simple rule which users encounter frequently and amend
      it with an obscure exception.

      If it is felt that direct derivation of an abstract non-interface type
      from an an interface is an important capability that must be
supported,
      then alternative syntax should be considered. Even an interminable
      string of keywords like

          type T is abstract new Some_Interface_Type
                    and not interface with null record;

      would be preferable to the AI's proposal; perhaps the
      "and not Interface" part could be omitted in the case where
      a non-empty list of components is given.

   6) It is intended that if two homographs are inherited as primitive
      operations of a tagged type, then they can share one slot in the
      dispatch table.

   7) No formal abstract interface types. Use formal abstract tagged private
      (or formal derived) instead. A type declared in the generic package
spec
      can't implement a formal tagged private, but that's ok. The situation
      today with deriving a non-abstract type from a formal abstract
      type is already quite ugly (the constract model does not include any
way
      to specify the set of abstract operations that must be overridden, so
      that check is deferred until the instantiation) - we don't want to
shine
      a spotlight on this corner of the language.

   8) The rule that only a tagged type may implement interfaces is not
      stated explicitly; it is implicit in the syntax.

   9) There are interactions between this AI and AI-279. In particular, it
is
      ok for Some_Abstract_Interface_Type'Class'Input to read in the tag of
a
      type which implements the interface but is not derived from it. The
rule
      given in AI-279 requires that Constraint_Error be raised in this case.
      This check should be relaxed.

   10) A primitive subprogram is inherited or it is not; it does not matter
if
       there is more than one reason for its inheritance - only one
inherited
       subprogram results.
       Does this need to be stated explicitly?

   11) The term "implement" is defined differently than in the AI. An
interface
       does not implicitly implement itself (because then it would inherit
       operators from itself).

   12) Nothing here (in particular, nothing in the 8.3 stuff) is intended to
       change the semantics of a program which declares no interface types.

   13) An example to illustrate some of the 8.3 stuff:

           package P1 is
               type T1 is abstract interface;
               procedure P (X : T1) is abstract; -- P'1
           end P1;

           package P2 is
               type T2 is abstract new P1.T1; -- P'2 (implicit)
               procedure P (X : T2) is abstract; -- P'3
           end P2;

           type D is new Some_Tagged_Type and P1.T1 and P2.T2 with null
record;

       D inherits procedures P'4, P'5, and P'6, corresponding to
       P'1, P'2, and P'3, respectively.

       Since P'3 overides P'2, P'6 overrides P'5 by the "overriding is
       preserved by inheritance" rule.

       Since P'2 corresponds to P'1,  P'5 overrides P'4 by the "inherited
       copies of your parent's ops hide inherited copies of your
grandparent's
       ops" rule.

       The word "simultaneous" is used in contradistinction to the word
       "previous" in 8.3(12). The word "corresponds" is used as
       "corresponding" is used in 3.4(17).


   14) Is this section of the proposed 8.3 addition,

           If one subprogram overrides another and a type simultaneously
           inherits subprograms corresponding to both, then the inherited
           subprogram corresponding to the overriding subprogram overrides
           the other inherited subprogram.

       , necessary, or can it be derived from existing language rules?

   15) Allowing implementation of a limited interface by a non-limited
       type might be feasible, but it would introduce some complications:

           A (dispatching) caller of a primitive function of a
           limited interface type returning that type would not know
           statically whether the (non-abstract) callee has a
           return-by-reference result type. For replicated-generic
           implementations, this would be something new.

           The caller of such a function would need to
           finalize the function result if and only if the function call
           introduces an anonymous object (see 7.6.1(13/1))); this
           would no longer be known statically.

       Strictly from a user's perspective (i.e. ignoring any implementation
       problems), would support for this construct even be desirable?

       The problems associated with disallowing this construct in the
       face of limited views of non-limited types (i.e. a limited
       formal type where the corresponding actual type is non-limited)
       are no worse than for 3.7(1)'s rule prohibiting access discriminants
       for non-limited types.

   16) Does the list of legality rules which need to be checked in
       the private part of an instance (see 12.3(11), AARM 12.3(11.y)) need
       to be extended? (The change to 3.9.1(3) is one such extension;
       the last paragrah of 3.9.4 is another).

   17) Recommended level of support for representation items for
       interface types = "confirmation only" ? Or perhaps no
       representation items at all for interface types - only
       operational items; would this be a legality rule or just the
       recommended level of support?

   18) Should the AI include a sample implementation model for
       My_Interface_Type'Class'Input ?

   19) Are there interactions between interface types and
       unknown discriminants that need to be addressed?
       I don't see any.

   20) Yes, it really is illegal if the parent type of a "with private"
       type's completion happens to implement some interface that the
       private view did not. This may turn out to be a pain.

       Relaxing the rule that a partial view must implement all interfaces
       implemented by the completion is not out of the question, but it
could
       open the door for some very peculiar situations. If this example,

        package P is
          packge Pkg is
              type Ifc is abstract interface;
              procedure Foo (X : Ifc) is abstract;
          end;

          type Parent_1 is tagged null record;

          type T1 is new Parent_1 with private;
        private
          type Parent_2 is new Parent_1 and Pkg.Ifc with null record;
          procedure Foo (X : Parent_2); -- Foo #1

          type T1 is new Parent_2 with null record;
        end;

        with P;
        package P_Client is
          type T2 is new P.T1 and P.Pkg.Ifc with null record;
          procedure Foo (X : T2); -- Foo #2
          X : T2;
        end P2_Client;

        with P_Client;
        package body P is
          ...
        begin
          Pkg.Foo (Pkg.Ifc'Class (P_Client.X));
             -- call Foo #2
          Pkg.Foo (Pkg.Ifc'Class (T1 (P_Client.X)));
             -- call Foo #1
        end P2;

      , were legal (it is illegal because the completion of T1 implements
      an interface that is not implemented by the the partial view), then
      we would have two dispatching calls to Pkg.Foo with the
      two controlling operands having the same tag and yet different
      bodies would be executed. The two conversions to Pkg.Ifc'Class
      would map Pkg.Foo to different slots in the same dispatch table
      because the source types of the conversions are different.

****************************************************************

From: Randy Brukardt
Sent: Thursday, May 16, 2002  11:31 PM

> Unreserved keywords (see AI-284):

Are you taking over this AI? It needs a discussion section at least.


>     Throughout the RM:
>         Replace "reserved word" with "keyword" (e.g. 3.9.3(2)).

NO, NO, NO! At the meeting, Bob pointed out that there are 96 occurrences of
this term in the standard. We can't make 94 text changes in the Amendment,
people will reject it as being too large without even seeing what's new.
Besides, it would take me at least 6 hours to do this, and I think I'd
demand extra pay to compensate for the extreme boredom involved. (Each
paragraph would have to be cut from the standard and properly formatted.)
Plus the tools would need to be updated to handle many, many changes from a
single AI.

At the very least, you have to enumerate every place that needs to be
changed. That would be necessary to help ensure that I don't miss any.

The minutes say "Tucker suggests the age-old solution of defining "reserved
word" to be equivalent to "reserved keyword". In the more general case, we
simply say "keyword".

I'm not adverse to making this change in paragraphs that we are changing
anyway (there are already five such uses, four of which are new). But,
please, lets stick with what we decided. We simply don't have the budget to
spend on unnecessary massaging.

...
>     --------
>
>     3.9.3
>
>      Add after paragraph 6:
>
>          Corresponding rules apply for a type which inherits an abstract
>          subprogram by implementing an abstract interface (see 3.9.4,
3.4).

Oh-oh! Do we have to define "corresponding" here? :-) :-)

****************************************************************

From: Steve Baird
Sent: Friday, May 17, 2002   2:25 PM

> Are you taking over this AI?

No (or at least I didn't think that I was).

I just wanted to provide enough context so that I
could talk about the keyword INTERFACE in the
AI-00251 writeup.

> The minutes say "Tucker suggests the age-old solution of
> defining "reserved word" to be equivalent to
> "reserved keyword". In the more general case, we
> simply say "keyword".

Fine with me.

****************************************************************

From: Tucker Taft
Sent: Friday, May 17, 2002  10:52 AM

Looks good.  I would suggest we define "implement" more generally,
so that a derived type "implements" its parent type.  Then we can
define "cover", etc. in terms of "implement" without any special cases.

****************************************************************

From: Steve Baird
Sent: Monday, May 20, 2002   1:32 PM

This sounds like a good approach.

This could solve a problem with the initial proposal,
where explicit conversion is required in cases where
we probably want to allow implicit conversion.

Given this example

    type Ifc is abstract interface;
    type T is new Some_Tagged_Type and Ifc with null record;

    type Ifc_Ref is access all Ifc'Class;

    X : aliased T;
    Ptr1 : Ifc_Ref := Ifc'Class (X)'Access; -- legal
    Ptr2 : Ifc_Ref := X'Access; -- legal ?

, I believe that the initial proposal would  allow the
first use of the Access attribute, but not the second.

I think that we want to allow the second use.

****************************************************************

From: Tucker Taft
Sent: Monday, May 20, 2002  1:40 PM

Yes, I agree that Ifc'Class should "cover" T and its derivatives.

I believe that the current AI implies that, but that your
proposed wording does not.

****************************************************************

From: Steve Baird
Sent: Wednesday, June 5, 2002   3:49 PM

Tuck says:
> I would suggest we define "implement" more generally,
> so that a derived type "implements" its parent type.  Then we can
> define "cover", etc. in terms of "implement" without any special cases.

I agree that if type T implements some interface type Ifc, then we want to
define "cover" so that Ifc'Class covers T.

I'm not so sure about changing the definition of "implement".

It would be nice to use a generalized form of "implementation" for
determining operator inheritance - a type inherits operators from the types
that it implements, and from no others.

This rule would imply that a derived type inherits operators not only from
its parent, but from all of its parent's ancestors.

Typically this extra inheritance would make no difference (all the extra
inherited operators would be overridden), but this would be an incompatible
change if an ancestor has primitive subprograms that the parent lacks (this
can only happen in the untagged case):

    package Pkg is
      type T1 is null record;
      type T2 is new T1;
      procedure P (X : T1);
      type T3 is new T2; -- T3 should not inherit a P procedure
    end Pkg;

With that in mind, here is a revision of my previous attempt at wording
for AI-251.

  -- Steve

----------------------------------------------------------------
----------------------------------------------------------------

    3.4.1(9)

    Add after the first sentence:
      If a type T1 implements an abstract interface type T2 (see 3.9.4),
      then T2'Class also covers all types covered by T1'Class.

    --------

    4.5.2: Ignore the changes I proposed earlier; revert to the current RM
text.

    --------

    4.6:

    Ignore the changes I proposed earlier.

    Replace paragraphs 21-23 with:

      If the target type is tagged, then either:

        The operand type shall be covered by or descended from the target type; or

        The operand type shall be a class-wide type that covers the target type.

      If the target type is not included in any of the above five cases,
      there shall be a type that is an ancestor of both the target type and the
      operand type.

****************************************************************

From: Steve Baird
Sent: Thursday, June 13, 2002   3:52 PM

I intended that the "implements" relation be transitive, but I didn't word
the definition correctly.

In this example,

     type Ifc1 is abstract interface;

     type Ifc2 is abstract interface;

     type Ifc3 is abstract new Ifc1 and Ifc2 with null record;

     type T1 is tagged null record;

     type T2 is new T1 and Ifc3 with null record;

, T2 implements Ifc3 and Ifc3 implements Ifc2, so therefore T2 should
implement Ifc2.

I believe that the proposed wording should be fixed to handle this case.

In the new 3.9.4 section, replace
        A descendant of a type which names an interface type in an
        abstract_interface_list, or which is derived from an interface type,
        is said to "implement" the interface and all of its ancestors.
with
        A descendant of a type which names an interface type in an
        abstract_interface_list, or which is derived from an interface type,
        is said to "implement" the interface and any other interfaces
        implemented by that interface.


Comments?

****************************************************************

From: Steve Baird
Sent: Wednesday, September 25, 2002   1:12 PM

This is my attempt at incorporating the comments of the Vienna meeting
into AI-251.
Changes include:
   1) "abstract interface" => "interface" in both terminology and syntax.
   2) A derived type is never an interface type, as described in the
      Vienna meeting minutes. To get the effect of derivation, declare
      a second interface type which implements the first one.
   3) The notion of implicit declarations being declared "simultaneously"
      is replaced with declarations occurring implicitly "at the same point".
   4) Formal interface types are added.
   5) A different way of expressing the notion that if a routine inherits
      two null homographs or two abstract homographs, then there is
      really no ambiguity.
   6) Expand the definition of "cover" so that Some_Interface_Type'Class
      covers all types which implement the interface.
   7) Define the "implements" relation to be transitive.

I'm not at all sure that these are all improvements, but I believe/hope that
they reflect the consensus of the group.
See also the discussion points at the end.

  -- Steve

----------------


Interface Types
    3.2.1

    Add
        type_definition ::= ... | interface_type_definition
    --------
    3.4

    Add after paragraph 23:
        A type which implements an interface type (see 3.9.4) inherits
        subprograms from the interface type, in the same way as for a type
        derived from the interface type.
    --------
    3.4.1(9)

    Add after the first sentence:
        If a type T1 implements an interface type T2 (see 3.9.4),
        then T2'Class also covers all types covered by T1'Class.

    --------

    3.9.1

    In paragraph 2, replace
        record_extension_part ::= WITH record_definition
    with
        record_extension_part ::= WITH [interface_list AND] record_definition
    .

    Add between sentences 3 and 4 of paragraph 3:
         The accessibilty level (see 3.10.2) of a type which implements an
         interface type (see 3.9.4) shall not be statically deeper than
         that of the interface type.

    Add after sentence 1 of paragraph 4:
         An interface type shall not be
         implemented in a generic body if the interface type is declared
         outside that body.
    --------

    3.9.3

     Add after paragraph 6:

         Corresponding rules apply for a type which inherits an abstract
         subprogram by implementing an interface type (see 3.9.4, 3.4).

    --------
    3.9.4 Interface Types
        An interface type is an abstract tagged type intended for use
        in providing a restricted form of multiple inheritance.
        A tagged type may "implement" multiple interfaces, thereby allowing
        multiple views of objects of the type.
                                 Syntax
        interface_type_definition ::= [LIMITED] INTERFACE [WITH interface_list]
        interface_list ::= interface_subtype_mark {AND interface_subtype_mark}
                                 Legality Rules
        An interface type (also called an "interface") is a specific abstract
        tagged type that is defined by an interface_type_definition.

        An interface type shall have no components.

        All primitive operations of an interface type shall be abstract
        subprograms or null procedures.

        The type of a subtype named in an interface_list shall be an
        interface type.

        A descendant of a type which names an interface type in an
        interface_list, or which is derived from an interface type,
        is said to "implement" the interface and any other interfaces
        implemented by that interface.
        A class-wide type implements the interfaces implemented by the
        corresponding specific type. The corresponding full view of a
        partial view of a type implements all interfaces implemented by
        the partial view.

        A type which implements an interface shall be limited if and only
        if the interface type is limited.

        If a partial view defines an interface type, then the corresponding
        full view shall define an interface type.

        If a full view implements an interface, then the corresponding
        partial view (if any) shall implement the interface.

        For an interface type declared in a visible part, a primitive
        subprogram shall not be declared in the private part.

        In addition to the places where Legality Rules normally apply
        (see 12.3), these rules apply also in the private part of an instance
        of a generic unit.

   --------

    4.6

    Replace paragraphs 21-23 with:

      If the target type is tagged, then either:
        The operand type shall be covered by or descended from the target type; or
        The operand type shall be a class-wide type that covers the target type.

      If the target type is not included in any of the above five cases,
      there shall be a type that is an ancestor of both the target type and the
      operand type.

    --------

    7.3

    Replace
        private_extension_declaration ::=
          type defining_identifier [discriminant_part] is
          [ABSTRACT] NEW ancestor_subtype_indication WITH PRIVATE;
    with
        private_extension_declaration ::=
          type defining_identifier [discriminant_part] is
          [ABSTRACT] NEW ancestor_subtype_indication
          WITH [interface_list AND] PRIVATE;

    --------
    8.3

    Add after paragraph 12:
        An implicit declaration of an inherited subprogram which is neither
        abstract nor a null procedure overrides that of a subprogram
        inherited by implementing an interface which is implicitly
        declared at the same point.

        If one subprogram overrides another and a type inherits subprograms
        corresponding to both which are implicitly declared at the same point,
        then the inherited subprogram corresponding to the overriding
        subprogram overrides the other inherited subprogram.

        If an inherited subprogram corresponds to another subprogram and a
        type inherits subprograms corresponding to both which are implicitly
        declared at the same point, then the subprogram corresponding to the
        inherited subprogram overrides the other inherited subprogram.

        If a type which implements an interface inherits two or more
        fully conformant abstract subprograms, or two or more fully
        conformant null subprograms, which are implicitly declared
        at the same point, and if the preceding rules would not specify
        any overriding relationships among these subprograms, then
        the implementation shall select one of these subprograms and it
        shall override the others.

    --------

    12.5

    Add
        formal_type_declaration ::= ... | formal_interface_type_definition
    --------

    12.5.1

    Replace
        formal_derived_type_definition ::= [ABSTRACT] NEW subtype_mark
                                           [WITH PRIVATE]
    with
        formal_derived_type_definition ::= [ABSTRACT] NEW subtype_mark
                                           [WITH [interface_list AND] PRIVATE]


    Add after paragraph 18:

        The actual type shall implement any interfaces that are implemented by
        the formal type.

    --------
    12.5.5 Formal Interface Types

    The class determined for a formal interface type is the class of all
    interface types.


                                 Syntax
        formal_interface_type_definition ::= interface_type_definition

                                 Legality Rules
    The actual type shall implement any interfaces that are implemented by
    the formal type.
    The actual type shall be limited if and only if the formal type is limited.

    --------

    13.14

    Add after paragraph 7:

        The implementation of an interface causes freezing of the
        interface type.

    --------

Discussion:
   1) A derived type is never an interface type. One can derive from an
      interface type, but the result is not an interface. Instead, define
      a new interface type which implements the "parent" interface.
      Similarly, a classwide type is never an interface type.

   2) We try to avoid having one section of the manual override another.
      Is the definition
          An interface type (also called an "interface") is a specific abstract
          tagged type that is defined by an interface_type_definition.
      "bad" in that sense? See 3.9.2(2) (the definition of a tagged type)
      and 3.9.3(2) (the definition of an abstract type).

   3) It is intended that in the case where the implementation
      arbitrarily chooses one overrider from among group of inherited
      subprograms, users should be unable to determine which
      member was chosen.

      In particular, users should be unable to do this by means of the
      Access (or Unchecked_Access) attribute.

      In order to accomplish this, and for other reasons, 4.5.2(13)'s rules
      about access-to-subprogram equality need to be loosened further to allow
      implementation-dependent results in the case of comparing access values
      which denote distinct null subprograms.

      That change is outside of the scope of this AI; it belongs in
      the AI in which null procedures are defined.

      Note that, formally speaking, the Address attribute poses no such
      problems. Although the requirements of the marketplace may be much
      more restrictive, the language itself allows an implementation
      tremendous freedom with respect to Address attribute values
      (see AI-173, 13.7.2(5)).

   4) 12.5.5 refers to "the class of all interface types". Is there a
      problem with having a class which is not closed with respect to
      derivation? As noted above, a derived type is never an interface type.

   5) There is no need to explicitly state that an interface type
      shall not implement itself (just as circular derivations are
      disallowed). This is a consequence of the rule that a partial
      view of an interface must implement any interfaces which are
      implemented by the full view.

   6) Recommended level of support for representation items for
      interface types = "confirmation only" ? Or perhaps no
      representation items at all for interface types - only
      operational items; would this be a legality rule or just the
      recommended level of support?


****************************************************************

From: Robert A. Duff
Sent: Thursday, September 26, 2002  6:28 PM

> This is my attempt at incorporating the comments of the Vienna meeting
> into AI-251.
...
> I'm not at all sure that these are all improvements, but I believe/hope that
> they reflect the consensus of the group.

Well, it might be useful to hear your objections...

Nitpick: You use "which" where I would use "that" in most cases.
(All except after ",".)

You use "point" where the RM uses "place".

> Discussion:
>
>    1) A derived type is never an interface type. One can derive from an
>       interface type, but the result is not an interface. Instead, define
>       a new interface type which implements the "parent" interface.
>       Similarly, a classwide type is never an interface type.

But interface'Class is legal, right?

>    2) We try to avoid having one section of the manual override another.
>       Is the definition
>           An interface type (also called an "interface") is a specific
>           abstract tagged type that is defined by an
>           interface_type_definition.
>       "bad" in that sense? See 3.9.2(2) (the definition of a tagged type)
>       and 3.9.3(2) (the definition of an abstract type).

There was a definite attempt in RM95 to put *all* the rules for abstract
things together in one section.  It wouldn't hurt to preserve that.

>    3) It is intended that in the case where the implementation
>       arbitrarily chooses one overrider from among group of inherited
>       subprograms, users should be unable to determine which
>       member was chosen.
>
>       In particular, users should be unable to do this by means of the
>       Access (or Unchecked_Access) attribute.
>
>       In order to accomplish this, and for other reasons, 4.5.2(13)'s rules
>       about access-to-subprogram equality need to be loosened further to allow
>       implementation-dependent results in the case of comparing access values
>       which denote distinct null subprograms.

I never liked that rule, but I guess you're right.

>       That change is outside of the scope of this AI; it belongs in
>       the AI in which null procedures are defined.
>
>       Note that, formally speaking, the Address attribute poses no such
>       problems. Although the requirements of the marketplace may be much
>       more restrictive, the language itself allows an implementation
>       tremendous freedom with respect to Address attribute values
>       (see AI-173, 13.7.2(5)).

Right.

>    4) 12.5.5 refers to "the class of all interface types". Is there a
>       problem with having a class which is not closed with respect to
>       derivation? As noted above, a derived type is never an interface type.

Why is it not closed under derivation?  Oh, I see, a type derived from
an interface is not an interface.  Well, yeah, it's bothersome to call
that a "class".  I don't see an easy way around it.

>    5) There is no need to explicitly state that an interface type
>       shall not implement itself (just as circular derivations are
>       disallowed). This is a consequence of the rule that a partial
>       view of an interface must implement any interfaces which are
>       implemented by the full view.

Could use a NOTE.

>    6) Recommended level of support for representation items for
>       interface types = "confirmation only" ? Or perhaps no
>       representation items at all for interface types - only
>       operational items; would this be a legality rule or just the
>       recommended level of support?

I'm not sure what you're getting at here.  Interfaces have no
components, so what's the "representation" issue.  Sure, I guess
operational items should be allowed.... Is it necessary to say anything?

****************************************************************

From: Tucker Taft
Sent: Friday, September 27, 2002  4:12 PM

> This is my attempt at incorporating the comments of the Vienna meeting
> into AI-251.
> ...
>     --------
>     3.9.4 Interface Types
>         An interface type is an abstract tagged type intended for use
>         in providing a restricted form of multiple inheritance.
>         A tagged type may "implement" multiple interfaces, thereby allowing
>         multiple views of objects of the type.
>                                  Syntax
>         interface_type_definition ::= [LIMITED] INTERFACE [WITH interface_list]
>         interface_list ::= interface_subtype_mark {AND interface_subtype_mark}
>                                  Legality Rules
>         An interface type (also called an "interface") is a specific abstract
>         tagged type that is defined by an interface_type_definition.

This is not a legality rule.  It is static semantics (or simply a definition).

>
>         An interface type shall have no components.

This is not a legality rule, so "shall have" => "has".
This is the static semantics (since there is no way to specify
components, it is silly to make this a legality/"shall" rule).

>         All primitive operations of an interface type shall be abstract
>         subprograms or null procedures.
>
>         The type of a subtype named in an interface_list shall be an
>         interface type.
>
>         A descendant of a type which names an interface type in an
>         interface_list, or which is derived from an interface type,
>         is said to "implement" the interface and any other interfaces
>         implemented by that interface.
>         A class-wide type implements the interfaces implemented by the
>         corresponding specific type. The corresponding full view of a
>         partial view of a type implements all interfaces implemented by
>         the partial view.

More static semantics (no "shalls" in these).

> ...

****************************************************************

From: Randy Brukardt
Sent: Friday, September 27, 2002  8:21 PM

Steve Baird wrote:

> This is my attempt at incorporating the comments of the Vienna meeting
> into AI-251.

...

Humm, I don't see any syntax for declaring that an original tagged type
implements an interface. There is only syntax for declaring that on an
extension. That doesn't seem right, why should it be necessary to declare an
extra type in order to say that a new tagged type implements some interface?
It also means that a private type which implements an interface cannot be
completed by a tagged type that is not an extension.

    type Root is tagged Window and record ...;

I would expect a syntax like:
    record_type_definition ::= [[abstract] tagged] [limited] [interface_list
and] record_definition

...

>   In order to accomplish this, and for other reasons, 4.5.2(13)'s rules
>   about access-to-subprogram equality need to be loosened further to allow
>   implementation-dependent results in the case of comparing access values
>   which denote distinct null subprograms.
>
>   That change is outside of the scope of this AI; it belongs in
>   the AI in which null procedures are defined.

What are you talking about? THIS is the AI where null procedures are defined!
The ARG did not decide to split those out of this AI. We would do that only
if there was a substantial chance that this AI would not be adopted while
null procedures would be.

---

It would also be valuable if the !proposal summary and !discussion sections were
updated to reflect the current proposal. They are getting way out of date,
and I'm not sure if the implementation models, etc. still apply to this
current wording.

****************************************************************

From: Tucker Taft
Sent: Sunday, September 29, 2002  5:29 PM

You just use the type extension syntax,
but make the parent type be the interface.
This is desirable so that a "normal" abstract tagged
type may be changed into an interface
type without changing any other code.

As with "abstract", a type is not an
interface unless the word "interface" appears
in its definition.  Deriving from an interface
does not make the derived type an interface.
In fact, derived types are never interfaces.
Interfaces are always considered "root" types
that happen to implement other interfaces.

****************************************************************

From: Randy Brukardt
Sent: Monday, September 29, 2002  6:44 PM

Humm, that would work, but it would be completely inconsistent and unexpected.
Moreover, what you would expect to do has no sensible way to indicate the
correct thing to do. Indeed, the RM would be completely devoid on any
indication of how to do this (as the derivation rules completely follow from
existing language rules, and aren't being modified).

My "mental image" of these things is that they are a new kind of item. If you
want a type to "implement" an interface, you put the interface into an
appropriate interface_list. That is not true in the case of derivation.

Moreover, deriving from an interface gives a incorrect view of the
(single-inheritance) type hierarchy. Such an derivation makes it appear that
the new type is a member of the interface type. That's simply wrong, if the
intent is to create a new hierarchy of types that happens to implement some
interface.

The fact that deriving these give a non-interface type is OK by me, but I view
that is something that exists at all simply to avoid contract model problems
with interfaces. Essentially, deriving one of these has to do SOMETHING, but
its not something that you would ever do in new code. Yes, it's helpful in
converting abstract types to interfaces, but I don't see that as something that
will happen often (for two reasons: "if it ain't broke, don't fix it", and
because most abstract types have concrete primitive operations, simply because
they could).

We'll have to dicuss this at the meeting.

****************************************************************

From: Tucker Taft
Sent: Monday, September 30, 2002  8:34 PM

I consider it essential that interface types and
abstract tagged types are very nearly interchangeable,
and that you can go back and forth relatively
easily, so long as the interface type is the first
one listed after "new".

The meaning of 'Class, "covers", implements, etc., should
all be such that it makes very little difference in
"type NT is new T with ..."
whether T is an interface, or an abstract tagged type.

At least I see that as an important goal.

****************************************************************

From: Randy Brukardt
Sent: Monday, September 29, 2002  10:03 PM

> I consider it essential that interface types and
> abstract tagged types are very nearly interchangeable,
> and that you can go back and forth relatively
> easily, so long as the interface type is the first
> one listed after "new".

I don't think this will be possible most of the time, because interfaces allow
multiple inheritance, and abstract types allow concrete operations and
components. Usually one or the other will be a requirement.

> The meaning of 'Class, "covers", implements, etc., should
> all be such that it makes very little difference in
> "type NT is new T with ..."
> whether T is an interface, or an abstract tagged type.

There certainly is value in keeping the proposal simple by reusing the abstract
rules in the usage of these things. And, as I said, I really don't care what
derivation means. What I object to is being required to make some random
interface "more important" than the others by being required to derive from it
when declaring the root of a type hierarchy. Just because some type happens to
implement an interface doesn't make the interface a parent of the type
hierarchy.

Indeed, one could argue that is the problem with Controlled types. (However,
they can't be an interface because they do have components - at least they do
in Janus/Ada. Illustrating that point.)

****************************************************************

From: Michael F. Yoder
Sent: Thursday, October 24, 2002  9:05 AM

We can get the benefit of Eiffel's renamings, but without awful syntax,
if we allow interface renamings to tweak things in a manner similar to
procedure renamings changing parameter names.  Something like:

   interface T2 renames T1 is
      procedure Name_Collision_On_P (args) renames P;
      function Oops_F_Has_Homonyms (args) return blah renames F;
   end interface;

Then, change the rule so that homonyms from different interfaces are
always illegal. If the user wants to complete two or more homonyms the
same way, she can use renamings-as-bodies to complete those she's forced
to rename.

****************************************************************

From: Michael F. Yoder
Sent: Thursday, October 24, 2002  9:31 AM

Sorry, I didn't include enough context for this to make sense to those
who didn't happen to be at the last meeting. The context is that a
tagged type is inheriting from two or more interfaces that have
homonyms, so the effect of blending them together isn't clear. There are
two cases:

(1) The homonyms aren't all mode-conformant to one another: one body
can't complete all of them.
(2) The homonyms are all mode-conformant, so completing them all is at
least possible.

There's probably uses for this ability other than avoiding name
collisions, e.g.:

   interface Sunviews_Style_Interface renames X_Style_Interface is...

****************************************************************

From: Pascal Leroy
Sent: Thursday, October 24, 2002  9:33 AM

I see that you believe that the proposal is not complex enough already ;-)

I mean, this AI is already extremely complicated, both from the language
design standpoint and from the implementation standpoint.  So complex in
fact that I doubt if it will make it into the final amendment.  And even if
it does, speaking with my implementer hat, I have a hard time believing that
we will make the vast investment needed to support it in our products (do I
sound like Robert?).

So I think we should strive to simplify the proposal, not add bells and
whistles that solve cornercase issues.

****************************************************************

From: Pascal Leroy
Sent: Thursday, October 24, 2002  10:03 AM

I agree that this proposal is way past the complexit threshhold. I don't
know if it will make it into the amendment, but I can be pretty sure
that folks at ACT are not going to give this one any attention regardless.

The danger of course is that if we get piles of proposals of this kind,
then a strong argument can and will be made to leave the whole thing
alone rather than rock the boat with over-complex stuff.

****************************************************************

From: Michael F. Yoder
Sent: Thursday, October 24, 2002  1:21 PM

>>I see that you believe that the proposal is not complex enough already

I thought the proposal was horridly complex also until I sorted through
what it actually meant. Also, much apparent complexity came from keeping
multiple variations in mind at the same time, only one of which can
actually be used for a final proposal.

I'm not a big OOP fan, but having a halfway-house technology gets the
worst of both worlds: OOP advocates will sniff at it, but you still have
the maintenance costs. If that sounds like a "keeping up with Java"
argument, well, it probably *is* that sort of an argument. I think
exception types fall into the same category.

The one instance where I might disagree with Tucker is that I'd prefer
that the line between the classes having treeish inclusion and the
others (the interfaces) be kept visible, and I would cheerfully buy
simplicity by making it more visible.

Oh, and finally, I don't expect the issue to be a cornercase issue in
practice unless interfaces just aren't used. If it were a cornercase
issue, why would the awful syntax of Eiffel's renaming conventions (for
solving this problem) ever occur? But perhaps Bob Duff can elaborate
about the circumstances in which he saw these things.

****************************************************************

From: Tucker Taft
Sent: Thursday, October 24, 2002  1:33 PM

I think this is much more important for Eiffel
because they use inheritance for *everything*
(including with/use clauses).  Note that Java
has no renaming capability, and they are succeeding
hansomely.  Also, Eiffel has multiple *implementation*
inheritance, whereas our proposal is a better fit for
Java, which has multiple *interface* inheritance.
In Eiffel, you have *code* that you are inheriting
potentially from two places, and perhaps that means you
more frequently will need to resolve conflicts.  With the proposed
interfaces, no code is being inherited, which might
simplify doing "renaming by editing" ;-).

My bottom line is I really don't think renaming
is important to making this useful, and Java
is my best evidence.  In Ada, we get to use
result type and defaulted parameters to distinguish
as well, whereas in Java, they don't have that
flexibility.  (Using defaulted but ignored parameters
to distinguish things might be a clever hack around
name conflicts, for those who really find them serious.)

****************************************************************

From: Robert A. Duff
Sent: Thursday, October 24, 2002  3:57 PM

> But perhaps Bob Duff can elaborate
> about the circumstances in which he saw these things.

I now believe that the name-clash problem should not be solved.
(I argued the opposite at the meeting, but Tucker's speech on the
subject convinced me to change my mind.)

I think this interfaces AI and the other AI about type stubs are the two
most important AI's on the table.  People really do choose other
languages over Ada because of these two major issues.  My evidence is of
course anecdotal, as is nearly all evidence in the so-called "science"
of Computer Science.  I have not done market surveys and the like.
Sigh.

I don't much care whether these two features make it into the Amendment,
but I think it's important to the long-term success of Ada that they get
implemented by the major compiler vendors, and in a uniform fashion (the
"uniform" part is what standards are all about!).

It worries me when I see Robert and Pascal threatening to not implement
these.  (I do understand the reasons.  SofCheck doesn't have tons of
funding to do this sort of thing, either, unfortunately.)

The "problem" we're discussing is that if you import two interfaces, and
get a name clash, you can get "stuck".  But the *usual* way to fix the
problem is to go edit the source code of one or both interfaces --
change the names so they don't clash.  The only time that won't work is
when you're importing two libraries, and you can't (or don't want to)
change their source.

The "problem" can only occur if the interfaces proposal is a great
success (widely implemented, and used by many vendors).  So I say:
let's solve that problem when it happens.  I.e. consider any "renaming"
bells and whistles as a totally separate AI from the basic interfaces
proposal.  There is no technical reason to solve the problem *now*.

I say, let's agree on a standard for the interfaces thing, and let's
implement it in the various compilers.  If and when Ada becomes wildly
successful, we can worry about the naming-clash problem.

Other points:

    - The naming clash problem really is a corner case: You have to have
      two independent developers choose the same name, the same number
      and types of parameters, the same result type, AND DIFFERENT
      MODES.

    - Any solution to the "problem" that I can think of is totally
      independent from the basic interfaces proposal -- so there's no
      reason to include it now.  As they say, let's burn that bridge
      when we come to it.

    - The interfaces proposal should be kept as simple as possible, so
      that vendors may find it feasible to implement on limited
      budgets.

    - Java doesn't address the "problem", and does just fine.

    - I don't agree with Mike that OOP advocates will "sniff at" the
      interfaces stuff without a solution to the naming clash problem.

For all these reasons, I am opposed to solving the naming clash problem
as part of this AI.  There's no harm in creating another AI to address
the problem, but *that* AI should be voted in 2015 or so.

****************************************************************

From: Robert A. Duff
Sent: Thursday, October 24, 2002  4:15 PM

> I thought the proposal was horridly complex also until I sorted through
> what it actually meant. Also, much apparent complexity came from keeping
> multiple variations in mind at the same time, only one of which can
> actually be used for a final proposal.

I agree with that.  The proposal represents non-trivial implementation
effort, but it's not *that* bad.  It always seems worse when you're
weighing 17 possible solutions to the same problem.

But I still say: let's keep it as simple as possible.
(But no simpler.  ;-))

****************************************************************

From: Michael F. Yoder
Sent: Thursday, October 24, 2002  6:11 PM

>- I don't agree with Mike that OOP advocates will "sniff at" the
>      interfaces stuff without a solution to the naming clash problem.

Just to set the record straight, I didn't say that; this is combining a
phrase into a different context. What I believe is that OOP advocates
will sniff at Ada if it has no multiple-inheritance mechanism that they
choose to regard as real. (There doesn't seem to be much enthusiasm for
the generic mixin mechanism et al., and I don't entirely blame them.)
Java seems to have established interfaces as real, and they don't break
the good properties (e.g. time-boundedness) of the single-inheritance
model, at least not for operations on the treeish classes.

****************************************************************

From: Robert Dewar
Sent: Thursday, October 24, 2002  7:20 PM

Hmmm, this seems dubious to me. Why do you believe this?

****************************************************************

From: Robert A. Duff
Sent: Thursday, October 24, 2002  4:15 PM

> Robert A Duff wrote:
>
> >- I don't agree with Mike that OOP advocates will "sniff at" the
> >      interfaces stuff without a solution to the naming clash problem.
> >
> Just to set the record straight, I didn't say that; this is combining a
> phrase into a different context.

Sorry if I misquoted you.

>... What I believe is that OOP advocates
> will sniff at Ada if it has no multiple-inheritance mechanism that they
> choose to regard as real. (There doesn't seem to be much enthusiasm for
> the generic mixin mechanism et al., and I don't entirely blame them.)
> Java seems to have established interfaces as real, and they don't break
> the good properties (e.g. time-boundedness) of the single-inheritance
> model, at least not for operations on the treeish classes.

I agree with that.  So the question is, will the lack of a solution to
the naming clash problem cause folks to scorn our multiple-inheritance
of interfaces solution?  I think not.

****************************************************************

From: Robert I. Eachus
Sent: Thursday, October 24, 2002  7:50 PM

>I now believe that the name-clash problem should not be solved.
>(I argued the opposite at the meeting, but Tucker's speech on the
>subject convinced me to change my mind.)

I think the problem is actually a bit larger than you think, but I don't
think that it is anything that the ARG, or WG9 can or should solve.
There are many cases where existing languages will see a clash and Ada
won't.  Great.  There is one case where Ada in my opinion should see a
problem and alert the user at compile time.  For example, let's say you
are inheriting from two different container types.  One has an Append
with an in out parameter, the parameter of the other is mode in.  Forget
for a moment all the philosophical and design issues which could result
in choosing one or the other.  There is a design problem here, and the
right thing for the compiler to do is to tell the user, not paper it
over.  If the user concludes that the two interfaces are not compatible?
He is right.  There are ugly workarounds, but they should be ugly.

The other case that hasn't been discussed as much, but has been
mentioned.  It is where you have say two Append procedures, one with a
single parameter and one that adds a position parameter with a default.
 It is possible to inherit from both interfaces, but the only way to
actually call the one parameter version is to rename it.  This is
currently one of the more unexpected ;-) features of Ada when it pops
up. But again there is no reason the ARG should try to "fix" anything.
If through generic instantiation and/or use clauses a call becomes
ambiguous for this reason, the programmer has to do real work to decide
between calls which should go to the one parameter version, and calls
which should use the default.  Shrug.   The compiler can only alert the
programmer to potential problems, it can't read his mind.

****************************************************************

From: Steve Baird
Sent: Tuesday, January 28, 2003 11:58 AM

This is my attempt at incorporating the comments of the Bedford meeting
into AI-251.

Changes include:
  1) New type declaration syntax.

  2) New derivation model (a type no longer "implements" an interface; it
     really is "derived" from the interface type, introducing a
     restricted notion of multiple parents for a derived type).

  3) New membership test, conversion rules.

  4) New homograph overriding requirements.

See also the discussion section at the end.

The last time I sent mail to the ARG list, it somehow ended up in
HTML format. I hope that problem has been straightened out,
but my apologies in advance if this message ends up similarly mangled.

  -- Steve

----------------



3.2.1 Type Declarations

Add to syntax

        type_definition ::= ... | interface_type_definition



3.4 Derived Types and Classes

Replace syntax section with

        interface_list ::= interface_subtype_mark {AND interface_subtype_mark}

        derived_type_definition ::=
            [ABSTRACT] NEW parent_subtype_indication
            [[AND interface_list] record_extension_part]


Add at the end of paragraph 3:

    A derived type has a parent type and zero or more interface parent types.



Replace paragraph 8 with:

    Each class of types that includes the parent type or an interface parent
    type also includes the derived type.

Add after paragraph 25:

    If a type declaration names an interface type in an interface list, then
    the declared type inherits any user-defined primitive subprograms of the
    interface type in the same way.

    Note: this includes some declarations of non-derived types.


3.4.1 Derivation Classes

Insert after the first sentence of paragraph 2:

     A derived type or interface type is also derived from each of its
     interface parent types, if any.

Replace the last sentence of paragraph 10 with:

    The ultimate ancestor of a type is the ancestor of that type, if any, that
    is not a descendant of of any other type and that is not an interface
    type (see 3.9.4).


3.9.3 Abstract Types and Subprograms

    Replace paragraphs 4-5 with

        If a type inherits a subprogram corresponding to an abstract
        subprogram or to a function with a controlling result, then

           - If the inheriting type is abstract or untagged, the inherited
             subprogram is abstract.


3.9.4 Interface Types

    This section is entirely new.

        An interface type is an abstract tagged type intended for use
        in providing a restricted form of multiple inheritance.
        A tagged type may be derived from multiple interface types, thereby
        allowing multiple views of objects of the type.

                                 Syntax

        interface_type_definition ::= [LIMITED] INTERFACE [WITH interface_list]

                                 Legality Rules

        An interface type (also called an "interface") is a specific abstract
        tagged type that is defined by an interface_type_definition.

        An interface type shall have no components.

        All user-defined primitive subprograms of an interface type shall be
        abstract subprograms or null procedures.

        The type of a subtype named in an interface_list shall be an
        interface type.

        If a type declaration names an interface type in an interface list,
        then the accessibility level of the declared type shall not be
        statically deeper than that of the interface type; also, the declared
        type shall not be declared in a generic body if the interface type is
        declared outside that body.

        A descendant of an interface type shall be limited if and only
        if the interface type is limited.

        A full view shall be a descendant of an interface type if and only if
        the  corresponding partial view (if any) is also a descendant of the
        interface type.

        For an interface type declared in a visible part, a primitive
        subprogram shall not be declared in the private part.

        In addition to the places where Legality Rules normally apply
        (see 12.3), these rules apply also in the private part of an instance
        of a generic unit.

4.5.2  Relational Operators and Membership Tests

                                Name Resolution

        Two types are said to "potentially share descendants" either if one
        covers the other or if both are class-wide and the corresponding
        specific type associated with at least one is an interface type.

        The simple_expression shall resolve to be of a type that potentially
        shares descendants with the tested type.

4.6 Type Conversions

        If the target type is tagged then either:

            - The operand type shall be covered by or descended from the target
              type; or

            - The operand type shall be a class-wide type that covers the
              target type; or

            - The operand and target types shall potentially share descendants.

        If the target type is not included in any of the above five cases,
        there shall be a type that is an ancestor of both the target type
        and the operand type.


7.3 Private Types and Private Extensions

    Replace
        private_extension_declaration ::=
          TYPE defining_identifier [discriminant_part] IS
          [ABSTRACT] NEW ancestor_subtype_indication WITH PRIVATE;
    with
        private_extension_declaration ::=
          TYPE defining_identifier [discriminant_part] IS
          [ABSTRACT] NEW ancestor_subtype_indication
          WITH [interface_list AND] PRIVATE;


8.3 Visibility

    Add after paragraph 12:

        An implicit declaration of an inherited subprogram which is neither
        abstract nor a null procedure overrides that of a subprogram
        inherited from an interface type which is implicitly
        declared at the same point.

        If one subprogram overrides another and a type inherits subprograms
        corresponding to both which are implicitly declared at the same point,
        then the inherited subprogram corresponding to the overriding
        subprogram overrides the other inherited subprogram.

        If an inherited subprogram corresponds to another subprogram and a
        type inherits subprograms corresponding to both which are implicitly
        declared at the same point, then the subprogram corresponding to the
        inherited subprogram overrides the other inherited subprogram.


    Add after paragraph 26:

        If a descendant of an interface type inherits two homographs
        which are implicitly declared at the same point, then both shall
        be overridden.

        Note: this requirement may be impossible to satisfy if the homographs
        are not mode-conformant.

12.5 Formal Types

    Add to syntax

        formal_type_declaration ::= ... | formal_interface_type_definition



12.5.1 Formal Private Types

    Replace
        formal_derived_type_definition ::= [ABSTRACT] NEW subtype_mark
                                           [WITH PRIVATE]
    with
        formal_derived_type_definition ::= [ABSTRACT] NEW subtype_mark
                                           [WITH [interface_list AND] PRIVATE]


    Add after paragraph 15:

        The actual type shall be a descendant of every ancestor of
        the formal type.


12.5.5 Formal Interface Types

    This section is entirely new.

        The class determined for a formal interface type is the class of all
        interface types.

                                 Syntax

        formal_interface_type_definition ::= interface_type_definition


                                 Legality Rules

        The actual type shall be a descendant of every ancestor of
        the formal type.

        The actual type shall be limited if and only if the formal type is
        limited.

13.14 Freezing Rules

    Add after paragraph 7:

        The declaration of a specific descendant of an interface type freezes
        the interface type.

--------

Discussion:

1) The compatibility rules for membership testing could be tightened to
   filter out more cases where the result of a membership test could not
   possibly be True. The two types could be required to agree with respect to
   limitedness, accessibility level, and/or set of enclosing generic bodies
   (with a recheck generic spec rule as well). Do we want all this in a
   name resolution rule? I'd say no.

2) "Potentially share descendants" seems like an awkward term.
    How about "weakly compatible"? "Potentially intersecting"?
    "Potentially convertible"? The idea here is that we want to allow any
    membership tests where the answer is not known statically. This includes
    a case like

       type T1 is tagged null record;
       type T2 is interface;

       X1 : T1'Class := ... ;
       X2 : T2'Class := ... ;

       Flag : Boolean := (X1 in T2'Class) and (X2 in T1'Class);

   because of the possibility of a type which is descended from both
   T1 and T2, e.g.
       type T3 is new T1 and T2 with null record;
   .

3) It might be possible to express the legality rules for type conversions more
   concisely. Would it be equivalent to replace the 3 listed conditions with
            - The operand type descended from the target type; or
            - The operand and target types shall potentially share descendants.
   ?

4) What representation attributes does an interface type have?
   Can values for these attributes be specified?
   What do T'Size and T'Alignment mean for any abstract type, not
   just for interface types?

5) Although an interface type which has an interface parent "is derived from"
   that type, it is not "a derived type".

6) Inherited homographs must be overridden. If the homographs are
   not mode-conformant, then this may be impossible. Life's hard.

7) Nothing here (in particular, nothing in the 8.3 stuff) is intended to
   change the semantics of a program which declares no interface types.

8) An example to illustrate some of the 8.3 stuff:

           package P1 is
               type T1 is interface;
               procedure P (X : T1) is abstract; -- P'1
           end P1;

           package P2 is
               type T2 is interface with P1.T1;  -- P'2 (implicit)
               procedure P (X : T2) is abstract; -- P'3
           end P2;

           type D is new Some_Tagged_Type and P1.T1 and P2.T2 with null record;

       D inherits procedures P'4, P'5, and P'6, corresponding to
       P'1, P'2, and P'3, respectively.

       Since P'3 overides P'2, P'6 overrides P'5 by the "overriding is
       preserved by inheritance" rule.

       Since P'2 corresponds to P'1,  P'5 overrides P'4 by the "inherited
       copies of your parent's ops hide inherited copies of your grandparent's
       ops" rule.

9) Yes, it really is illegal if the parent type of a "with private"
   type's completion happens to be descended from some interface that the
   private view was not descended from. This may turn out to be a pain.

   Relaxing the rule that a partial view must be derived from the same
   interfaces as the completion is derived from is not out of the question, but
   it could open the door for some very peculiar situations. If this example,

        package P is
          packge Pkg is
              type Ifc is interface;
              procedure Foo (X : Ifc) is abstract;
          end;

          type Parent_1 is tagged null record;

          type T1 is new Parent_1 with private;
        private
          type Parent_2 is new Parent_1 and Pkg.Ifc with null record;
          procedure Foo (X : Parent_2); -- Foo #1

          type T1 is new Parent_2 with null record;
        end;

        with P;
        package P_Client is
          type T2 is new P.T1 and P.Pkg.Ifc with null record;
          procedure Foo (X : T2); -- Foo #2
          X : T2;
        end P2_Client;

        with P_Client;
        package body P is
          ...
        begin
          Pkg.Foo (Pkg.Ifc'Class (P_Client.X));
             -- call Foo #2
          Pkg.Foo (Pkg.Ifc'Class (T1 (P_Client.X)));
             -- call Foo #1
        end P2;

   , were legal (it is illegal because the completion of T1 is descended
   an interface that the partial view is not descended from), then
   we would have two dispatching calls to Pkg.Foo with the
   two controlling operands having the same tag and yet different
   bodies would be executed. The two conversions to Pkg.Ifc'Class
   would map Pkg.Foo to different slots in the same dispatch table
   because the source types of the conversions are different.


--------

In defining interface types, a couple of design decisions seem to be
of interest.

First, there is the question of whether interface types should
be a whole new class of type or a special kind of abstract tagged
type. Since interface types have to abide by all of the restrictions
of an abstract tagged type, the former approach would have involved
a lot of repetition. The latter approach was chosen.

Second, there is the question of derivation and operator inheritance.
If a type "implements" an interface, then it must inherit operators
from the interface type in order to override them. In an earlier version
of this AI, the "implementing" type was not considered to be derived from the
parent interface type and operator inheritance was accomplished via
a separate mechanism. Now, a type which implements an interface type
is defined to be derived from the interface type, thereby simplifying the
operator inheritance rules and the definitions of terms such as
"ancestor", "descendant", and "cover". This means that a given type may
be immediately derived from more than one type, but at most one of them
can be a non-interface type. It is no longer the case that every type
has an "ultimate ancestor", but the term remains well-defined in all the
cases where it is used in the reference manual.

The rules for type conversion and membership testing need revision to
cope with the possibility that an interface type and a tagged type
might have a common descendant even though statically available information
suggests no relationship between the two types.

As an implementation model, one can think of a type which
inherits operations from an interface type as defining a
mapping from the primitive operations of the interface type (perhaps
represented as dispatch table slot numbers) to those of the inheriting type.
A dispatching call to an operation of an interface type is than accomplished
by mapping the the called operation to the corresponding
operation of the controlling operand's type. Alternatively, one
might construct a separate dispatch table representing this
alternative "view" of the "real" dispatch table, but that would
require more work when deriving from a type which implements
an interface. The slot-to-slot mapping remains valid and can be
reused for the derived type, whereas a new alternative "view"
of the dispatch table of the derived type would need to be
constructed. A value of type Some_InterFace_Type'Class can then
be represented at runtime as a record address paired with a
reference to this dispatch table permutation map.

Classwide streaming operations for interface types require
some care. Consider the case of a call to Some_Interface_Type'Class'Input
occuring as the controlling operand of a dispatching call. The
implementation must be able to determine where to dispatch to in this case.
Membership testing may also require maintaining additional information
at runtime.

It is intended that a type which is derived from an interface type should
inherit the same subprograms as a type which is derived from both the
interface type and an ancestor of the interface type. The two type definitions
should be indistinguishable, in the sense that adding or deleting the
explicit redundant derivation relationship should be a semantics-preserving
transformation.

It is intended that there should be no real difference between the two
forms of derivation from interface type parents. For example, replacing
the declaration
   type T is new Interface_Type_1 and Interface_Type_2 with null record;
with
   type T is new Interface_Type_2 and Interface_Type_1 with null record;
should be semantics-preserving.

---------

An example involving interface types:

   type T is tagged null record;
   T_Var : T;

   package P1 is
      type Ifc1 is interface;
      procedure Op    (X : Ifc1) is abstract;
      procedure Op1_A (X : Ifc1) is abstract;
      procedure Op1_B (X : Ifc1) is abstract;

      type Ref is access all Ifc1'Class;
      Procedure Foo (X : Ref);
   end;

   package P2 is
      type Ifc2 is interface;
      procedure Op    (X : Ifc2) is abstract;
      procedure Op2_A (X : Ifc2) is abstract;
      procedure Op2_B (X : Ifc2) is abstract;

      type Ref is access all Ifc2'Class;
      Procedure Foo (X : Ref);
   end;

   package body P1 is
      ...

      procedure Foo (X : Ref) is
         type Stream_Ptr is access all Ada.Streams.Root_Stream_Type'Class;
         The_Stream : constant Stream_Ptr := ...;
      begin
          Op1_A (X.all);
          Op1_B (X.all);
          Ifc1'Class'Output (The_Stream, X.all);

          if X.all in T'Class then
              T_Var := T (X.all);
          end if;
      end Foo;
   end P1;

   package body P2 is
      ...
   end P2;

   type D is tagged
       with P1.Ifc1 and P2.Ifc2 and
       record
           F1, F2 : Integer;
       end record;

   procedure Op1_A (X : D);
   procedure Op1_B (X : D);
   procedure Op    (X : D);
   procedure Op2_A (X : D);
   procedure Op2_B (X : D);

   ...

   X : aliased D;
 begin
   P1.Foo (X'Access);
   P2.Foo (X'Access);
   P1.Op (P1.Ifc1'Class (X));
   P2.Op (P2.Ifc2'Class (X));
 end;

****************************************************************

From: Steve Baird
Sent: Tuesday, May 13, 2003  3:59 PM

AI-251 introduces null procedures. Their dynamic semantics are
specified as follows:

    The execution of a null procedure is invoked by a subprogram call.
    This execution has no effect.

This seems wrong. This wording should be replaced with

    The execution of a null procedure is invoked by a subprogram call.
    The execution of the subprogram_body of a null procedure has no
effect.
.

RM 6.3.2(10) breaks the execution of a subprogram call down into several
steps. Calling a null procedure is different than calling a non-null
procedure
only with respect to the step "The subprogram body is then executed.". The
other steps (evaluation of the name or prefix of the call, parameter
evaluation,
and parameter copy-back) are performed in the same way as for a non-null
procedure.

Elaboration checking is not an issue here because a null procedure cannot
have a forward declaration.

****************************************************************

From: Robert I. Eachus
Sent: Friday, June 13, 2003  3:31 AM

There is nothing all that exciting here--I hope--but it does seem worth
capturing.  First, this shows one way of "fixing" the naming clash
problem.  And it really is a good programming in the large approach.  An
underlying type may be bound to a dozen or more interfaces, but changes
to one binding won't affect the others, adversely or otherwise.

Second, it brings up an interesting issue.  In this example, List may be
an interface type, but the Iterator type is in reality part of the
interface.  There is no problem implementing that part of the interface,
but it would be nice to have a way to specify that type Iterator, and
two operations on the type that don't have parameters of type List are
still part of the List interface, and need to be provided by any type
which implements the interface.

This is actually a much more general problem.  I'd say that as many as
50% of the types that would make reasonable interface types have some
sort of "helper" type declarations in the package that defines the
interface.   Arbitrarily deciding that all the other types declared in a
package that declares an interface type are part of the interface in
some way might work, but then there would be the problem of binding
existing types to one of these helper types.

And this particular way of creating iterators shows why this can be painful.

=========================================================================
Alexander Kopilovitch wrote:

It seems to be an unfortunate collision that multiple new packages
should be proposed when Abstract Interfaces still aren't established
firmly. It is highly likely that some packages (for example, Data
structures components) may look significantly better if they can use
Abstract Interfaces.

I personally don't see any conflict.  The interface AI will allow easier
bindings to C++ and Java.  But in Ada, mix-ins are a better abstraction
IMHO for containers.  The advantage is that you can easily put objects
in a container even if the original declarer of the type/class had no
idea that they would be put in a container.  For example:
...

Yes, but what about similar containers, such as various flavors of List?
And it is not an easy task to align properly the above your words with
another your opinion (with which I fully agree), posted here about 3
weeks ago - I mean the following:

...  In Ada you tend not to have one sort
package in your toolbox, or one list type implementation, you have
several.  Now the programmer solving some problem sees a part of his
decomposition that can be solved by a list package or a sort package,
and does a generic instantiation.  The problem is that there is no easy
way, in Ada, to express a binding to a member of the class of sort
generics, but to delay the choice of the actual mapping.

This is why one of the features I expect interfaces to add to Ada is a
better way of organizing collections of sort algorithms and the like.

Real subtle point.  Interfaces as currently planned will ALLOW multiple
interface inheritance.  They can also be used as I described in the
first paragraph to provide multiple instances of a single abstract
interface.  I expect the multiple (interface) inheritance use to be
common when interfacing to code written in other OO languages.  The
usage I described is almost not a programming convention but a way to
describe the relationship between a number of otherwise unrelated
abstractions.  They all implement the same interface, so they can be
passed to generics as instances of that interface, but otherwise there
is no necessary relation between the abstractions.

Can you use multiple interface inheritance to describe abstractions that
match more than one interface?  Sure, and it will happen.  For example
imagine an AVL_Tree package which clearly allows random (indexed)
access, and also has an ability to do an inorder walk.  It can also
implement the list interface.  The question is whether it will be
common.  The problem becomes a programming in the large issue.  It is
going to be much easier to create a list 'view' of the AVL_Tree package
and keep it consistant with the list interface, than to maintain a
package that directly implements multiple interfaces.

This is probably a very good example, so let me follow it through a bit.

    generic
      type Element is private;
    package Lists is
      type List is abstract interface;
      function Head(L: in List) return Element;
      function Is_Empty(L: in List) return Boolean;
      procedure Append(L: in out List; E: in Element);

      type Iterator is private with null;

      function Create(L: in List) return Iterator;
      function Next(I: in Iterator) return Element;
      function At_End(I: in Iterator) return Boolean;

    private
      ...
    end Lists;

I did the iterator this way because it brings up an interesting question
about interface types.  Obviously an instance of the Lists package also
creates a new Iterator type, and that type is closely related to type
List.  But are functions like Next and At_End part of the List
interface?  I think it is an issue AI-251 needs to address.

Now I want to create a binding between this interface and a tree type.
The package spec is easy:

    with Lists;
    generic
      with package Trees is new AVL_Trees;
      -- Could make the element type explicit, but no reason to, it is
      -- already specifed by the element type in the AVL_Trees instance.
      -- (And by Elements.Element.  They had better match.)
    package List_View is

      type List is private with null;

      function Head(L: in List) return Element;
      function Is_Empty(L: in List) return Boolean;
      procedure Append(L: in out List; E: in Element);

      type Iterator is private with null;

      function Create(L: in List) return Iterator;
      function Next(I: in Iterator) return Element;
      function At_End(I: in Iterator) return Boolean;

    private
      type List is Trees.AVL_Tree;
      type Iterator is Trees.Inorder_Walk with null;
    end List_View;

Now it gets tough.  Not because it is hard to write, but their is one
tough decision.  Head is fairly easy to write, start at the root of the
tree and follow left links until you find one that is null.  The
contents of that node are what you want to return.  Is_Empty and the
Iterator operations shouldn't be too hard, since I specified that
AVL_Trees provides an inorder walk interator.  But what to do about
Append?  I see three options:

1) Always raise Program_Error.  In other words, this is just a view, and
not a full list implementation.

2) Raise an error if the new Element does go at the end of the list.
Err, tree.  This allows the Append operation to be used to insert a
sorted set of Elements one at a time.

3) Insert the Element at the proper position in the tree, and invalidate
any existing Iterators, that are past the position the new Element will
be inserted in, either in the implementation or by fiat.  (In other
words, document it as an error to insert with an open Iterator.)

Whichever solution you choose, all done.  Notice that we have created a
binding between one abstraction Lists, and another, AVL_Trees, that
never explicitly uses the new interface feature to implement multiple
inheritance.  We could have done it, but some of the List operations
only loosely make sense for an AVL_Tree viewed as a tree (the Head
operation) while others will be hidden by the interface entirely.  It
may be that some of the operations for the List view may perfectly match
the available operations for the AVL_Tree (Is_Empty?), but many will
have different names.  So what?  The "overhead" of making the list view
explicit makes maintenance much easier.

=======================================================================

Added for ARG:  Notice that if I make List_View.List an explicit
interface derived from Lists.List or more properly from an instance of
Lists.Lists, let's call in My_List, there will be a problem.  I get a
function:

function Create(L: in List_View.List) return My_Lists.Iterator;

That function is of no use to anyone.  But the operations Next and
At_End have no parameters (or return values) of type List.  So they are
not part of the (proposed Ada) interface.  However, an Interface type,
and Create, Next, and At_End operations are a part of the interface from
a design point of view.  Do I have a solution?  Not yet.  I can actually
do what I want in current Ada--but without the multiple interface
inheritance.  It is trying to map an existing abstraction to a different
interface that brings these issues to light.

****************************************************************

From: Pascal Leroy
Sent: Monday, June 16, 2003  2:10 AM

> Added for ARG:  Notice that if I make List_View.List an explicit
> interface derived from Lists.List or more properly from an instance of

> Lists.Lists, let's call in My_List, there will be a problem.  I get a
> function:
>
> function Create(L: in List_View.List) return My_Lists.Iterator;
>
> That function is of no use to anyone.  But the operations Next and
> At_End have no parameters (or return values) of type List.  So they are
> not part of the (proposed Ada) interface.

I don't see why you claim that the above function is "of no use to
anyone".  It creates a perfectly good iterator.  The annoyance is that,
to play with this iterator, you have to use operations from both
List_View and My_Lists.  This happens often and I agree that it can be
pretty confusing.  However, I have a feeling that the Object.Operation
notation would help tremendously here:

	Iter : My_Lists.Iterator;
	L : List_View.List;
	...
	Iter := L.Create;
	while not Iter.At_End loop
		...
		Iter.Next;
	end loop;

In fact, I was not too excited about the Object.Operation notation, but
I find this case quite compelling, as the programmer is relieved from
having to find the "right" package.

****************************************************************

From: Stephen W Baird
Sent: Tuesday, September 2, 2003  4:02 PM

This version of AI-251 reflects the feedback of the Toulouse meeting.

Changes include:
    1) Null procedures are no longer defined in this AI; they have their
       own AI now.
    2) Subprogram overriding rules are revised.
    3) Membership test legality rules are defined in terms of
       convertibility.
    4) The term "interface ancestor" replaces the term "interface
       parent".
    5) The discussion section and the examples are updated.

[This is version /08 of the AI - ED]

****************************************************************

From: Robert I Eachus
Sent: Tuesday, September 2, 2003  8:45 PM

Stephen W Baird wrote:

>        A descendant of an interface type shall be limited if and only
>        if the interface type is limited.

I want to be sure I understand this rule and how it interacts with
generic instantiation.  It is currently common to define a generic with
a limited type parameter, if the generic does not depend on assignment
or equality for the type.  I'm worried about the "only if:" part of the
rule here.  Can a decendant of an interface type be passed as a generic
formal parameter of a limited type?

The problem case of course is if the generic actually derives a type
from the generic parameter:

generic
   type Foo is limited private;
   ..
package Bar is

   type Foobar is new Foo...;
   ...
   function Foob return Foobar;

end Bar;

package My_Bar is new Bar(Foo:
Some_Non_Limited_Type_Descended_From_An_Interface);

But according to your explanation:

8) Someone asked why a limited interface cannot be implemented by a
   non-limited type. An implementation would no longer know statically
   whether a dispatching function call is a call to a function with a
   return-by-reference result. This could be implemented, but it seems
   like an unnecessary complication. The language has, as a matter of
   general principle, steered away from constructs which would allow a
   limited view of an object of a non-limited tagged type.

There should be no problem, because in the instance, type Foobar will be
non-limited if Some_Non_Limited_Type_Descended_From_An_Interface is also
non-limited.  So even if an object of type Foobar is passed to some classwide
operation for Some_Non_Limited_Type..., or one of its parents, there should be
no problem.

****************************************************************

From: Tucker Taft
Sent: Sunday, September 28, 2003  9:51 AM

I have had real trouble keeping the syntax
for interface types in my head.  Every time
I write an example, I get something wrong.
I just took a look at AI-251, and I can see
why ;-).  We aren't being consistent, and
I suspect I am at least partly to blame.

Here is the inconsistency I find most mystifying:

     type T is new Q and R and S with record ... end record;

     type T is new Q with R and S and private;

I certainly think of "with record ...;" and "with private;"
as similar constructs.  I feel strongly these two need
to be reconciled somehow.

And here is the form for one interface inheriting from others:

     type P is interface with Q and R;

(no "new" even though this is a "derived" interface).

I am now trying to figure out what to propose
for the syntax for inheriting from protected and
task interfaces.  Here are two possibilities:

     protected type T is new <interface_list> with ...
   or:
     protected type T with <interface_list> is ...

and then there is the "derived" interface case:

     protected interface PI is new <interface_list> with ...
    or:
     protected interface PI with <interface_list> is ...


Any comments/suggestions?

I *think* I would prefer always putting interfaces
after the word "with", or never doing so.  The half-and-half
approach is too confusing and hard to remember.

I also feel some preference for having "new" in any
type that is derived from something else.  As it is
now, derived interfaces don't have "new" anywhere.

A radical suggestion would be to put the word "interface"
out front:

    interface Q;

    interface Q is new R and S;

In any case, I found the current proposal a bit of a
confusing mish-mosh, and the protected/task interface
proposal seems likely to make it worse...

****************************************************************

From: Tucker Taft
Sent: Sunday, September 28, 2003  12:04 PM

> A radical suggestion would be to put the word "interface"
> out front:
>
>    interface Q;
>
>    interface Q is new R and S;
> ...

This would be treating "interface" as a
substitution for the term "type" instead of
as a kind of type.  This works better for protected
and task interfaces, since the syntax clearly
looks better with the word "type" *replaced*
with the word "interface" rather than writing something like:

    protected type interface PI is ...
   or
    protected interface type PI is ...

So maybe this radical suggestion has something
going for it, especially if we are considering
generalizing it to cover protected/task interfaces.

And it allows the use of "new" for derived
interfaces, which would also be more consistent
in my view.

****************************************************************

From: Vincent Celier
Sent: Sunday, September 28, 2003  12:13 PM

Ar you suggesting that these should be replaced with

     protected interface PI is ...

?

So,

     protected Interface is ...

would be a single_protected_declaration, and

     protected interface PI is ...

would be an interface protected type declaration.

I am not sure I would like that.

****************************************************************

From: Tucker Taft
Sent: Sunday, September 28, 2003  12:31 PM

> Ar you suggesting that these should be replaced with
>
>      protected interface PI is ...
>
> ?

Yes.

>
> So,
>
>      protected Interface is ...
>
> would be a single_protected_declaration, and
>
>      protected interface PI is ...
>
> would be an interface protected type declaration.
>
> I am not sure I would like that.

Good point.  I think here we would disallow the
use of "Interface" as the name of a singleton protected
(or task) object, even if interface is a non-reserved keyword.
That seems like an acceptable incompatibility.

****************************************************************

From: Pascal Leroy
Sent: Monday, September 29, 2003  2:02 AM

>Here is the inconsistency I find most mystifying:
>
>     type T is new Q and R and S with record ... end record;
>
>     type T is new Q with R and S and private;
>
>I certainly think of "with record ...;" and "with private;"
>as similar constructs.  I feel strongly these two need
>to be reconciled somehow.

I agree that this needs fixing.  As a matter of fact, I see this discrepancy as
a bug in the AI.  It appears that the syntax for private extension comes from
the Vienna meeting, but at the subsequent Bedford meeting we changed the order
of things, and came up with the syntax for record extension, above.  Evidently
the AI was not updated consistently.

>And here is the form for one interface inheriting from others:
>
>     type P is interface with Q and R;
>
>(no "new" even though this is a "derived" interface).

Again this was discussed several times, and as a matter of fact I find it
useful that there is no "new" here, as the syntax makes it clear that you are
not declaring a derived type, but only composing interfaces.

>I am now trying to figure out what to propose
>for the syntax for inheriting from protected and
>task interfaces.  Here are two possibilities:
>
>     protected type T is new <interface_list> with ...
>   or:
>     protected type T with <interface_list> is ...

The first syntax is the one that is consistent with the Bedford decision.

>and then there is the "derived" interface case:
>
>     protected interface PI is new <interface_list> with ...
>    or:
>     protected interface PI with <interface_list> is ...

I am concerned by the possible lookahead problems for parsers: when you reach
the word "interface", you don't know if it's a keyword or an identifier.
Furthermore, this syntax is inconsistent with the interface composition syntax
above.

>Good point.  I think here we would disallow the
>use of "Interface" as the name of a singleton protected
>(or task) object, even if interface is a non-reserved keyword.
>That seems like an acceptable incompatibility.

This doesn't make sense at all.  The sole purpose of unreserved keywords is to
avoid incompatibilities.  As you know, unreserved keywords are already a
contentious issue.  Unreserved keywords with incompatibilities are sure to be
shot down at the WG9 level.  For my part, I am going to oppose any AI that has
such a blatant incompatibility anyway.

****************************************************************

From: Tucker Taft
Sent: Monday, September 29, 2003  5:38 AM

P> I agree that this needs fixing.  As a matter of fact, I see this
P> discrepancy as a bug in the AI.  It appears that the syntax for private
P> extension comes from the Vienna meeting, but at the subsequent Bedford
P> meeting we changed the order of things, and came up with the syntax for
P> record extension, above.  Evidently the AI was not updated consistently.

OK, that makes me feel better.
Though I frankly couldn't remember which we settled
on.  I take it that it was the first of the two above.

>     I am now trying to figure out what to propose
>     for the syntax for inheriting from protected and
>     task interfaces.  Here are two possibilities:
>
>          protected type T is new <interface_list> with ...
>        or:
>          protected type T with <interface_list> is ...
>
P> The first syntax is the one that is consistent with the Bedford decision.

OK.

>     and then there is the "derived" interface case:
>
>          protected interface PI is new <interface_list> with ...
>         or:
>          protected interface PI with <interface_list> is ...
>
P> I am concerned by the possible lookahead problems for parsers: when you
P> reach the word "interface", you don't know if it's a keyword or an
P> identifier.  Furthermore, this syntax is inconsistent with the interface
P> composition syntax above.

Well, there really isn't any ambiguity, provided you are
willing to look ahead a bit.  I believe Vincent was more worried
about conceptual ambiguity.  However, I now believe that
if we do decide to have non-reserved keywords, then we must
not worry about conceptual ambiguity, but rather rely on
programmers to have good sense.  In all of our proposals it
is possible to make things look funny by using the non-reserved
keyword as an identifier in the same construct where the
non-reserved keyword is usable as a keyword.

>     Good point.  I think here we would disallow the
>     use of "Interface" as the name of a singleton protected
>     (or task) object, even if interface is a non-reserved keyword.
>     That seems like an acceptable incompatibility.
>
P> This doesn't make sense at all. ...

I agree with you.  We need to accept that the non-reserved
keyword proposal is subject to the possibility of (intentional?)
confusion, but so be it.  As you point out, it is solely
a compatibility technique, so introducing incompatibilities
*and* non-reserved keywords makes little or no sense.

I also realized after I replied that "protected type interface Blah is ..."
has exactly the same problem.  That is, "protected type Interface is ..."
would be a type, whereas "protected type interface Blah is ..."
would be an interface.

There also would be nothing preventing someone from writing
"type Interface is interface;" or, perhaps worse,
"type Interface is tagged;" and other weird-o combinations.

P> The sole purpose of unreserved keywords
P> is to avoid incompatibilities.  As you know, unreserved keywords are
P> already a contentious issue.  Unreserved keywords with incompatibilities
P> are sure to be shot down at the WG9 level.  For my part, I am going to
P> oppose any AI that has such a blatant incompatibility anyway.

I hear you.  The proposal does *not* have these incompatibilities.
They appeared only in my quick, and ill-conceived I realize now,
response to Vincent.

You made no comment on the idea of using "interface" as a
*substitute* for the word "type" in the syntax.  Any
comments on that?

****************************************************************

From: Robert A. Duff
Sent: Monday, September 29, 2003  5:43 AM

> This doesn't make sense at all.  The sole purpose of unreserved keywords
> is to avoid incompatibilities.  As you know, unreserved keywords are
> already a contentious issue.  Unreserved keywords with incompatibilities
> are sure to be shot down at the WG9 level.  For my part, I am going to
> oppose any AI that has such a blatant incompatibility anyway.

The potential confusion between "Interface is" and "interface Mumble is"
is no big deal, in my opinion.  If you get it wrong, you get a
compile-time error message, which is relatively harmless.

Useful compiler options would be to warn about uses of unreserved
keywords as identifiers, or to treat them as reserved words.
Let's leave that sort of thing to compiler options.

****************************************************************

From: Pascal Leroy
Sent: Monday, September 29, 2003  7:49 AM

> OK, that makes me feel better.
> Though I frankly couldn't remember which we settled
> on.

That's why we have meeting minutes.  They contain really useful
information, and are make very pleasurable reading, so I think you
should look at them more often. ;-)

> >          protected interface PI is new <interface_list> with ...
>
> Well, there really isn't any ambiguity, provided you are
> willing to look ahead a bit.

You're right of course, and from that perspective it is quite different
from the ambiguity that Vincent pointed out (and which we must avoid at
all costs).  However, I am still concerned that this could wreak havoc
in our LALR(1) parser.

> There also would be nothing preventing someone from writing
> "type Interface is interface;" or, perhaps worse, "type
> Interface is tagged;" and other weird-o combinations.

And we should ignore such oddities.  Yes, you can write really bizarre
Ada code if you try hard enough, but programmers who write code that way
get what they deserve.  We should be concerned about helping readability
for folks who write reasonable code.

> You made no comment on the idea of using "interface" as a
> *substitute* for the word "type" in the syntax.  Any
> comments on that?

I don't like the idea of using "interface" as a _substitute_ for "type",
because I would like to see the word "type" whenever we declare a type.
That seems like a useful readability invariant.  However, if you look at
the Bedford minutes, you'll see:

"Pascal suggests: interface type I2 is new I1 and I2 and I0;"

So I am not opposed to moving "interface" at the beginning of the
declaration.

****************************************************************

From: Robert Dewar
Sent: Monday, September 29, 2003  8:42 AM

> I am concerned by the possible lookahead problems for parsers: when you
> reach the word "interface", you don't know if it's a keyword or an
> identifier.  Furthermore, this syntax is inconsistent with the interface
> composition syntax above.=20

A little lookahead limited to a few tokens is no big deal, if necessary
the lexer can do this if you are stuck with a parser with no lookahead
capability. I would not distort designs too much at this level of concern.

****************************************************************

From: Robert A. Duff
Sent: Monday, September 29, 2003  2:40 PM

> That's why we have meeting minutes.  They contain really useful
> information, and are make very pleasurable reading, so I think you
> should look at them more often. ;-)

Yes, they're at least as amusing as the comics in The Boston Globe.
Perhaps I should start reading these minutes to my 10-year-old son at
bed time, instead of the Isaac Asimov robot stories he demands these
days.

> > >          protected interface PI is new <interface_list> with ...
> >
> > Well, there really isn't any ambiguity, provided you are
> > willing to look ahead a bit.
>
> You're right of course, and from that perspective it is quite different
> from the ambiguity that Vincent pointed out (and which we must avoid at
> all costs).  However, I am still concerned that this could wreak havoc
> in our LALR(1) parser.

Seems like you could make it work.  I mean, at least the *lexer* can
look ahead?

> > There also would be nothing preventing someone from writing
> > "type Interface is interface;" or, perhaps worse, "type
> > Interface is tagged;" and other weird-o combinations.

..or "type Untagged_Nonlimited is tagged limited..."

> And we should ignore such oddities.  Yes, you can write really bizarre
> Ada code if you try hard enough, but programmers who write code that way
> get what they deserve.  We should be concerned about helping readability
> for folks who write reasonable code.

I'll get up on my high horse now (basically agreeing with Pascal).  My
language design principle is *not* "Punish those who make bad programs."
My language design principle is: "Make it easy to make good programs!"
The former is futile.  The latter is hard work, but it's our job.

[Editor's note: Most of the following discussion is on the concept of
unreserved keywords, and is filed there (in AI-284).]

****************************************************************


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