Re: Parametrically polymorphic operations (Was: Safe C++)

[Wes Groleau <wwgrol@sparc01.ftw.rsc.raytheon.com>]

Jacob Sparre Andersen wrote:
>
> 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 "*"?

Any conforming compiler.  To continue your example:

  Result := Matrix_A * Matrix_B;

If the function returns a four by five matrix, constraint_error
results.  This is of course at RUN-TIME though.  However, IF the
compiler is able to determine at compile time that this WILL be
the case it is allowed to issue a warning and many will.  However,
it is rare that this can be determined at compile time.

For that reason, your extension would be very useful in a minority
of cases, where the bounds of all three matrices are known and
the body of "*" is not.  This is a big enough minority that
I'd like to see it evaluated for the next revision.  But there
may be other good reasons for avoiding it.

You may be able to do such a thing statically in Eiffel.

-- 
Wes Groleau
http://freepages.rootsweb.com/~wgroleau