Re: Interresting, possibly buggy behavior in GNAT generics w/ expression function.

[Eryndlia Mavourneen <eryndlia@gmail.com>]

On Friday, March 29, 2013 12:43:05 PM UTC-5, Eryndlia Mavourneen wrote:
> On Friday, March 29, 2013 11:21:39 AM UTC-5, Dmitry A. Kazakov wrote:
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> > On Fri, 29 Mar 2013 14:45:33 +0000, Simon Wright wrote:
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> > > "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
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> > >> On Fri, 29 Mar 2013 07:45:04 +0000, Simon Wright wrote:
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> > >>> I don't know whether this is a language problem
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> > >> It is a general language problem that generic specifications and bodies
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> > >> cannot be fully checked. Matched formal parameters only is not sufficient.
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> > >> Much better than with C++ templates, but still same mess.
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> > > But, do you think it should have failed at instantiation?
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> > It should fail to compile, instantiation time is too late. Generic
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> > contracts are much too weak.
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> > For example:
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> > generic
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> >    type S is new T with private;
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> > package P is
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> >    type Q is new S with null record;
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> >    not overriding procedure Foo (X : Q);
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> >    
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> > It is uncheckable if S does not have Foo already. Thus, this must be a
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> > compile error.
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> > If we wanted properly contracted generics (personally, I don't care), we
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> > would need some syntax to tell that S may not have operation Foo, e.g.
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> > generic
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> >    type S is new T with private;
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> >    not with procedure  Foo (X : S);
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> > package P is
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> >    type Q is new S with null record;
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> >    not overriding procedure Foo (X : Q); -- We know S does not have this
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> > -- 
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> > Regards,
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> > Dmitry A. Kazakov
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> > http://www.dmitry-kazakov.de
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> I agree that generics are too weak.  It seems that I frequently am trying to use a generic formal parameter's characteristics, but the compiler reminds me that the characteristic is unknown (until instantiation), and so the compilation of the generic fails.  Indices are a good example.  I am not sure about 'Range.  That might work.
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> -- Eryndlia

I do realize, of course, that indices can be specified as formal parameters, but that is a lot of hassle.  I seem to recall that there were other issues, too, that failed in compiling an generic unit.

-- Eryndlia