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From: Stuart Palin <stuart.palin@gecm.com>
Subject: Re: 'with'ing and 'use'ing
Date: 2000/03/09
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Organization: Avionic Systems, Marconi Electronic Systems
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Newsgroups: comp.lang.ada
Date: 2000-03-09T00:00:00+00:00
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Robert Dewar wrote:
> 
> In article <38C52D30.E1C7E85A@gecm.com>,
>   Stuart Palin <stuart.palin@gecm.com> wrote:
> 
> > It seems to me that the points raised in the c.l.a thread
> > "choice of fixed point type" highlight an area where the "use"
> > clause can give rise to some concerns.
> 
> Nope, you are misreading the thread. This thread was about

Quite possibly!

> a) ambiguities not detected that should be. Obviously adding
> use clauses NEVER causes undetected ambiguities.

True - because the compiler is supposed to report an error if there is
an ambiguity.

> b) the concern that certain expressions are compiled
> misleadingly because only one fixed-point type is
> visible. Use clauses can *only* help in this regard.

This is more the area I was concerned with.

First a caveat - I am working with Ada-83, and I do not know whether
what follows works
differently under Ada-95.

Suppose I (rather pathologically) have:
  package P1 is
    type Gx is range -1000..1000;
    function I return Gx;
    function K return float;
  end P1;

  package P2 is
    type Fx is delta 0.1 range -200.0 .. 200.0;
    function I return Fx;
    function K return Fx;
  end P2;

And then I start writing a new package:

  package P3 is
  end P3;

  with P1, P2; use P1,P2;
  package body P3 is
    X : float;
  begin
    -- Now I "intend" to do :  X := float(P1.I) * P1.K;

    -- But foolishly write:
    X := float(I * K);

    -- This will be compiled (under Ada-83) as :  X := float(P2.I *
P2.K);

  end P3;

If I had written (what I probably should have written if I did not
believe there was an
ambiguity due to P2 being in scope)

  X := float(I) * K;

then the compiler would have warned me about the ambiguity (at least of
I).

I believe it is a concern that a mistake can be hidden, because another
valid (but unexpected) meaning can be "found" by the compiler, that
leads people to "distrust" using "use".

Regards
--
Stuart Palin
BAE SYSTEMS Electronics Limited