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!standard 04.05.05(20)          04-02-29 AI95-00364/01
!class amendment 03-12-04
!status work item 04-02-29
!status received 03-09-29
!priority Medium
!difficulty Hard
!subject Fixed-point multiply/divide
!summary
This proposal attempts to alleviate the upward incompatibility created by Ada 95 for users defining their own fixed-point multiply or divide operators.
!problem
The pervasive visibility of universal-fixed multiply/divide operators coupled with the implicit conversion provided by Ada 95 resulted in an unforeseen incompatibility for users defining their own fixed-point multiply or divide operators. Essentially such operators are unusable, because the universal-fixed ones make any use of them ambiguous.
!proposal
Forbid use of universal-fixed multiply operator if both operands have at least one primitive user-defined multiply operator. Similarly, forbid use of universal-fixed divide operator if both operands have at least one primitive user-defined divide operator. These would be Name Resolution rules.
!wording
Change 4.5.5(20) as follows:
[Legality Rules] {Name Resolution}
The above two fixed-fixed multiplying operators shall not be used in a context where the expected type for the result is itself universal_fixed -- [the context has to identify some other numeric type to which the result is to be converted, either explicitly or implicitly]. {An explicit conversion is required on the result when using the above fixed-fixed multiplication operator when both operands are of types with user-defined primitive fixed-fixed multiplication operators. Similarly, an explicit conversion is required on the result when using the above fixed-fixed division operator when both operands are of types with user-defined primitive fixed-fixed division operators.}
{AARM NOTE: We have made these into Name Resolution rules (one of them existed in Ada95 but as a Legality Rule) to ensure that user-defined primitive fixed-fixed operators are not made unusable due to the presence of these universal fixed-fixed operators.}
!discussion
After taking a shot at defining the universal-access equality operators in standard, it became obvious that there is a simple fix for the universal-fixed incompatibility. Just have a name-resolution rule which forbids use of the universal-fixed operations if both operand types have at least one primitive user-defined multiply operator, in the case of the univ-fixed multiply op, or both have at least one primitive user-defined divide operator in the case of the univ-fixed divide op.
This doesn't have any beaujolais effects, because it isn't a preference rule. Just certain fixed-point type combinations can't be used with the universal-fixed multiply/divide operators, based strictly on whether they both have their own primitive multiply/divide operators, even where these primitive operators are not visible.
Note that we still allow the universal-fixed operators to be considered if there is an explicit conversion on the result. This is intended to provide Ada 83 compatibility. This implies that ambiguity is still possible, but only in the same places where ambiguity existed in Ada 83. A qualified expression on the result can be used to break the ambiguity in favor of the user-defined operator in such a situation, followed by the desired conversion. As in Ada 83, there is no way to make use of the universal-fixed operator if the operand types correspond to types for which there is a visible user-defined operator.
OPEN ISSUE:
I phrased the suggested rule as requiring "both" to have a user-defined primitive operator, but it could be phrased as "either." "Both" makes sense if multiple fixed-point types are defined in the same scope. "Either" makes sense if fixed-point types tend to be defined each in their own package, because the "first" one defined would likely not have any operators of its own. Compatibility with Ada 83 is enhanced by specifying "either." Compatibility with Ada 95 is enhanced by specifying "both." Because explicit conversion is always available as a fall back, perhaps "either" is preferable, since it has less dependence on the structure of the packages defining the fixed-point types.
This isn't a totally hypothetical problem. At one point we definitely dealt with a customer who was trying to figure out how to get their old Ada 83 fixed-point code to work, and the answer was not pretty. Basically they had to use non-operator functions like "mul" and "div", and live with the fact that they couldn't prevent misuse of the predefined univ-fixed operators.
There are various reasons to want to "opt out" of the universal-fixed operators. In some cases the user-defined operator performs wrap-around or "saturation" arithmetic. In other cases, there may be a desire to limit what combinations of fixed-fixed types are allowed, so that only those that involve no scaling are permitted. This may be for efficiency, or simply for "physical units" consistency.
With this rule, so long as each fixed point type has at least one primitive fixed-fixed "*" operator and one primitive fixed-fixed "/" operator, the univ-fixed operators will never be used.
!example
Here are three fixed-point types, with multiplication and division operators that require no scaling. These may be user-defined because they perform "saturation" arithmetic, or simply to ensure that they are only combined in ways that make sense.
type T1 is delta 0.1; type T2 is delta 0.001; type T3 is delta 0.0001; function "*"(Left : T1; Right : T2) return T3; function "*"(Left : T2; Right : T1) return T3; function "/"(Left : T3; Right : T1) return T2; function "/"(Left : T3; Right : T2) return T1;
X1, Y1 : T1 := ...; X2, Y2 : T2 := ...; X3, Y3 : T3;
begin X3 := X1 * X2; -- Does not use univ-fixed operator, no ambiguity X3 := X1 * X1; -- Error! Does not use univ-fixed operator, and -- no user-defined op works either X1 := X3 / X2; -- Does not use univ-fixed operator, no ambiguity X3 := X1 / X2; -- Error! Does not use univ-fixed operator, -- and no user-defined op works either X3 := X3 * X3; -- Error! Does not use univ-fixed operator (because -- X3 has a primitive which is fixed * fixed, even though -- X3 appears only as result type). X3 := T3(X3 * X3); -- OK, because of explicit conversion
--!corrigendum
!ACATS test
ACATS test(s) should be constructed to check these changes.
!appendix

From: Tucker Taft
Sent: Monday, September 29, 2003 10:53 PM

I indicated I had a solution for the ada 95/ada 83
incompatibility relating to fixed-point multiply/divide,
where user-defined multiply are not usable because
of the "pervasive" visibility of the universal-fixed
multiply/divide operations.

After taking a shot at defining the universal-access
equality operators in standard, it became obvious
that there is a simple fix for the universal-fixed
incompatibility.  Just have a name-resolution rule
which forbids use of the universal-fixed operations
if both operand types have at least one primitive user-defined multiply
operator, in the case of the univ-fixed multiply op,
or both have at least one primitive user-defined divide operator in
the case of the univ-fixed divide op.

This doesn't have any beaujolais effects, because it isn't
a preference rule.  Just certain fixed-point type combinations
can't be used with the universal-fixed multiply/divide
operators, based strictly on whether they both have their
own primitive multiply/divide operators.

[I phrased the suggested rule as requiring "both" to have
a user-defined primitive operator, but it could be
phrased as "either." ]

By the way, this isn't totally hypothetical.  At one
point we definitely dealt with a customer who was trying
to figure out how to get their old Ada 83 fixed-point code
to work, and the answer was not pretty.  Basically they
had to use non-operator functions like "mul" and "div",
and live with the fact that they couldn't prevent misuse
of the predefined univ-fixed operators.

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

From: Robert Dewar
Sent: Monday, September 29, 2003 10:55 PM

We have run into similar situations. I like this fix. I think it is
worth while. In fact I would suggest allowing Ada 95 compilers to do
this right away, why not?

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

From: Robert I. Eachus
Sent: Tuesday, September 30, 2003  3:45 PM


Tucker said:

> This doesn't have any beaujolais effects, because it isn't
> a preference rule.  Just certain fixed-point type combinations
> can't be used with the universal-fixed multiply/divide
> operators, based strictly on whether they both have their
> own primitive multiply/divide operators.

First let me say that this problem is MUCH more serious than potential
beaujolais effects.  A good solution now should be preferred to continuing to
look for a perfect solution.  There may be a clean and easy to implement
solution without beaujolais effects.  If so great.  But a preference rule NOW
would be great too.

> [I phrased the suggested rule as requiring "both" to have
> a user-defined primitive operator, but it could be
> phrased as "either." ]

There are three potential fixed-point types involved, two operands and a
result.  I think Tucker is talking about when the two operands both have user
defined multiply or divide operations.  I don't particularly care what the
exact rule is though, as long as you get it defined for all cases.

> By the way, this isn't totally hypothetical.  At one
> point we definitely dealt with a customer who was trying
> to figure out how to get their old Ada 83 fixed-point code
> to work, and the answer was not pretty.  Basically they
> had to use non-operator functions like "mul" and "div",
> and live with the fact that they couldn't prevent misuse
> of the predefined univ-fixed operators.

I have/had some fielded Ada 83 code on one project where this one issue was
enough to cause the project to stay with a "baselined" Ada 83 compiler.  I
don't know that the particular package ever got rewritten for Ada 95, but
distance times sine returning distance, and (apparent) speed divided by cosine
to give ground speed were two examples of where it was used in the code.

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

From: Robert I. Eachus
Sent: Sunday, February 29, 2004  11:12 AM

I prefer the "either" rule.  It would probably be possible to define a
'better' rule somewhere in the middle, but getting to complex in this
rule would certainly confuse users.  If any package exports both a
fixed-point type and a fixed-fixed "*" operation for it, calling the
predefined universal_fixed "*" is usually not wanted in any case, even
if the fixed-fixed operator is for say Dollars * Hours.

I will point out one surprising result of this rule.  By 3.2.3(6), an
operation in the private part of a package can be a primitive operation
for a (non-private) type declared in the public part of the package.
The right solution may just be "don't do that."

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

From: Robert Dewar
Sent: Monday, March  1, 2004  6:29 PM

Unless there is better motivation that Ada 83 compatibility, I would
forget this. It's rather beside the point to worry about this issue
at this stage, the damage is already done!

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

From: Randy Brukardt
Sent: Monday, March  1, 2004  7:34 PM

Humm, is this the same Robert Dewar who wrote a few months ago:

"We have run into similar situations. I like this fix. I think it is
worth while. In fact I would suggest allowing Ada 95 compilers to do
this right away, why not?"

:-)

Anyway, vendors have indicated that they have customers that have stuck with
Ada 83 precisely because of this issue. Removing barriers to migration to
newer versions Ada would seem to be valuable.

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

From: Robert Dewar
Sent: Monday, March  1, 2004  8:13 PM

> Humm, is this the same Robert Dewar who wrote a few months ago:
>
> "We have run into similar situations. I like this fix. I think it is
> worth while. In fact I would suggest allowing Ada 95 compilers to do
> this right away, why not?"

Yes, it's the same Robert Dewar (I wrote that in October 2003). I guess
the additional time passing has made me feel less sangine about the
effect at this stage.

> Anyway, vendors have indicated that they have customers that have stuck with
> Ada 83 precisely because of this issue. Removing barriers to migration to
> newer versions Ada would seem to be valuable.

Well that's a very strong and significant user input, which seems
decisive to me.

So I think on reflection I will go with the October RD rather than
the March RD :-)

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

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