!standard  12.07 (03)                                  02-10-06  AI95-00317/01
!class amendment
!status Amendment 200Y 02-10-06
!status work item 02-10-06
!status received 02-10-06
!priority Medium
!difficulty Low
!subject Partial Parameter Lists for Formal Packages

!summary

A formal package may have a partially specified set of actual parameters.

!problem

When a generic has more than one formal package parameter, it is often
important to link the two packages via their actuals.  However, the
only way to link the packages is to specify one with no actuals, and
specify the other will all of its actuals.  For example:

    generic
        with package Inst1 is new Gen1(<>);
        with package Inst2 is new Gen2(Inst1.A, Inst1.B, ...);
        ...
    package GP is ...

The problem is that there may be some actuals of the second formal
package which are *not* linked to the first one.  The only way to
deal with this is to add more formal parameters to the generic GP to
specify the values for the remaining actual parameters.  For example:

    generic
        type T1 is private;
        type T2 is private;
        with package Inst1 is new Gen1(<>);
        with package Inst2 is new Gen2(Inst1.A, Inst2.B, FT3 => T1, FT4 => T2);
        ...
    package GP is ...

Unfortunately, this approach defeats much of the advantage of formal
package parameters, which is to reduce the number of formals and simplify
the usage of a generic.

!proposal

A formal package parameter may specify some, but not all, of its actual
parameters.  For example:

    generic
        with package Inst1 is new Gen1(<>);
        with package Inst2 is new Gen2(Inst1.A, Inst2.B, others => <>);
        ...
    package GP is ...

The parameters which are not specified may be referenced by name as
expanded names using the formal package as its prefix.  For example,
Inst2.FT3 and Inst2.FT4 may be used later in the formal generic part
of GP or within the spec or body of GP.  By contrast, the specified
actuals may not be so named, to avoid confusion between the properties
of the actual and those of the formal.

Similarly, any implicit declarations associated with a given parameter
of the formal package may be named using the formal package as a
prefix may only when the actual for the parameter is *not* specified.

With this proposal, the notation "(<>)" for its actual part is essentially
a short-hand for "(others => <>)."

!wording

(*TBD*)

!discussion

This is intended to be a natural generalization of the two capabilities
currently provided for specifying actual parameters of a formal package.
By providing this "mid-point" where some but not all of the parameters
are specified, the formal package capability becomes significantly more
useful, without measurably increasing the complexity of supporting the
capability.

!examples

!ACATS test

!appendix

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