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Re: [Coq-Club] Eval compute and type classes


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  • From: Julien Tesson <julien.tesson AT univ-orleans.fr>
  • To: AUGER Cédric <sedrikov AT gmail.com>
  • Cc: coq-club AT inria.fr
  • Subject: Re: [Coq-Club] Eval compute and type classes
  • Date: Fri, 12 Oct 2012 15:50:59 +0200

Le 12/10/2012 15:27, AUGER Cédric a écrit :
> Le Fri, 12 Oct 2012 15:00:21 +0200,
> Julien Tesson
> <julien.tesson AT univ-orleans.fr>
> a écrit :
>
>> Le 11/10/2012 10:47, Arnaud Spiwack a écrit :
>>> Yes.
>>>
>>> It's occasionally useful to be able to compute with existential
>>> variables.
>>>
>>> Why is it strange ?
>> It looked strange for me at first because I would have expected the
>> same behaviour (in term of failure) between [Eval compute in a] and
>> [Definition x := Eval compute in a], and because when I use type
>> classes, I expect that the instance search mechanism warn me when it
>> failed to find a matching instance.
>>
>> This new behaviour seems interesting but I found it surprising at
>> first.
>>
>> Regards,
>> Julien
> I guess "Definition x := Eval compute in a" could be made to work by
> automatically generalizing existentials, buth then I fear it would
> become hard to predict what really will x be.
I'm not sure it would be a good idea.
In the cases where I use type classes, I really want that the definition
fails if there exists no instance which give a definition for the field I'm
using.



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