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From: riehler@ajpo.sei.cmu.edu (Richard Riehle)
Subject: Re: intermediate results causing constraint errors
Message-ID: <1994Dec6.030051.6818@sei.cmu.edu>
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Organization: AdaWorks Softwasre Engineering
References: <3bn3l9$pa2@gnat.cs.nyu.edu> <D0Cp98.1AK@inmet.camb.inmet.com>
Date: Tue, 6 Dec 1994 03:00:51 EST
Date: 1994-12-06T03:00:51-05:00
List-Id: <comp.lang.ada>

In article <D0Cp98.1AK@inmet.camb.inmet.com> bobduff@dsd.camb.inmet.com (Bob Duff) writes:
>In article <3bn3l9$pa2@gnat.cs.nyu.edu>, Robert Dewar <dewar@cs.nyu.edu> wrote:
>>Richard Riehl worries about "constraint errors in intermediate results
>>in expressions of mixed type"
>>
>>What's *that* about?
>>
>>Ada gives you complete control over the type of all intermediate results,
>
>Robert,
>
>I didn't see Richard Riehl's comment, but I don't understand your point.
>If I write:
>
>    type T is range 1..10_000;
>    X: T := 10_000;
>    Five: T := 5;
>    Ten: T := 10;
>    ...
>    X := (X * Five) / Ten;
>
>one implementation might overflow on the multiply and another might not.

   In fact, the Mil-Std 1815 ALRM explicitly alllows for this variation 
   in implementation by requiring Numeric_Error (not Constraint_Error)
   when run-time errors occur due to problems during evaluation of 
   intermediate expressions.  Check out the wording of ALRM 11.6 / 6.

    Richard Riehle 

>
>It's true that there are ways to avoid this (define a wider-ranged type
>first, and define T as a subtype of that), but I would say that that
>requires some "Ada experience".  Furthermore, the Ada compiler won't
>necessarily tell you about the problem, either at compiler or run-time
>-- you'll find out when you try to port the code from a 32-bit machine
>to a 16-bit machine (a rarity these days, I suppose).
    >
    One need not even restrict this  knotty little problem to inter-platform 
    porting.  I have used compilers which require some care in planning
    for this eventuality. 

     R. Riehle

>>and you only get constraint error if you choose the wrong type. It would
>>be wrong for *any* language not to give this kind of control. I simply
>>don't see *any* problem here, and certainly I don't see a problem that
>>needs the expertise of "experienced Ada" programmers to deal with it.
>
    Unfortunately, it is not a language issue.  The ALRM does have something
    to say about it.  But it is not forbidden by the ALRM.  And maybe that
    does make it a language issue.  This is one of those caveat emptor
    issues.  Ignore it at your peril  :-)

    Richard Riehle