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From: Jacob Sparre Andersen <sparre@nbi.dk>
Newsgroups: comp.lang.ada
Subject: Re: Parametrically polymorphic operations (Was: Safe C++)
Date: Fri, 21 Sep 2001 19:13:27 +0200
Organization: Centre for Chaos and Turbulence Studies, Niels Bohr Institute
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Date: 2001-09-21T19:13:27+02:00
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Wes:

> Jacob Sparre Andersen wrote:
> > safe types, I have certainly wanted something very similar
> > to parametrically polymorphic functions. It would make it
> > possible to have more extensive compile time checks, than
> > what is possible with Ada as it is defined today.
> >
> > An example:
> >
> >    type Matrix (Left, Right : Positive) is private;

Nobody seems to have noticed, but the type should of course
have been declared as:

   type Matrix (Width, Height : Positive) is private;

> >
> >    function "*" (Left  : in Matrix;
> >                  Right : in Matrix (Height => Left.Width))
> >      return Matrix (Width  => Right.Width,
> >                     Height => Left.Height);
> 
> Where are you going to put the result?

For example in:

   Result : Matrix (Width => 4, Height => 4);

> If the implementation is reasonable, constraint_error
> will be raised when the constraints on the left side don't
> the constraints on the result.

The implementation of my favourite Ada compiler or the
implementation of function "*"?

The problem is that the constaints on the result can not be
expressed in the specification of "*". You have to put them
in the implementation of "*". GNAT may not be a reasonable
implementation of an Ada compiler, but it is the one I use
most of the time, and I have not been able to get it to
complain about mismatches in the dimensions of matrices,
when doing matrix multiplications. I would appreciate it, if
you could explain us how to write a matrix multiplication
function, which allows some of the existing Ada compilers to
complain about mismatches in matrix dimensions at compile
time.

> And if the constraints are static on both, many compilers
> will detect it at compile time.

Would you care to mention one or two examples?

Jacob
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