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

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 29 Mar 2013 19:56:05 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
> news:1ex3dm2hk2j54$.1uiyljwziv0hi$.dlg@40tude.net...
> ...
>>> But, do you think it should have failed at instantiation?
>>
>> It should fail to compile, instantiation time is too late. Generic
>> contracts are much too weak.
> 
> This doesn't make much sense, as you then go on to suggest adding a formal 
> contract which would have the effect of rejecting an instantiation if the 
> type actually had Foo. That's exactly what Ada does, so I don't see any 
> advantage of a complication.

That is because you consider only one possibility, that is a client-side
error. Then instantiation is the right time to report this contract
violation. For the generic package itself this time is when the generic is
compiled.

My example had the purpose to show that matching formal parameters is not
enough. It could illustrate an error in the generic itself as well, e.g.
when *all* intended actual types for instantiations of P have Foo per
design.

> The problem you are having is that you are viewing the contract of a generic 
> in too narrow of a fashion. The contract includes not only the things in the 
> generic part, but also a number of properties expressed in the rest of the 
> specification.

You lumped together two different contracts. There is a contract on the
actual parameters which is defined by be the formal parameters
specification. Another contract is the generic package specification
(public part), which is a contract for the clients using an instance of the
package.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de