Fundep question

[Gabor Greif]

With *UndecidableInstances* switchd on it seems to work :-)

Thanks for the help!

    Gabor

Em sexta-feira, 31 de julho de 2015, Gabor Greif <ggreif at gmail.com>
escreveu:

> No. I'll switch that on and report back.
>
> Thanks,
>
>     Gabor
>
> Em sexta-feira, 31 de julho de 2015, Simon Peyton Jones <
> simonpj at microsoft.com
> <javascript:_e(%7B%7D,'cvml','simonpj at microsoft.com');>> escreveu:
>
>> you need "liberal coverage checking", so UndecidableInstances. Are you
>> doing that?
>>
>> |  -----Original Message-----
>> |  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of
>> |  Richard Eisenberg
>> |  Sent: 31 July 2015 16:10
>> |  To: Gabor Greif
>> |  Cc: ghc-devs
>> |  Subject: Re: Fundep question
>> |
>> |  Let's rewrite with explicit kind variables, noting that b is also
>> |  poly-kinded:
>> |
>> |  class Dep k k2 (a :: k) (b :: k2) | a -> b k2
>> |      -- if a determines b, it surely determines k2
>> |
>> |  instance Dep k * x y => Dep (Maybe k) * (Just x) (Maybe y)
>> |
>> |  Actually, even with the kinds explicit, it still looks valid to me.
>> |  Post a bug report?
>> |
>> |  Richard
>> |
>> |  On Jul 31, 2015, at 9:54 AM, Gabor Greif <ggreif at gmail.com> wrote:
>> |
>> |  > Hi all,
>> |  >
>> |  > say I want to instantiate
>> |  >
>> |  >    class Dep (a :: k) b | a -> b
>> |  >
>> |  > as
>> |  >
>> |  >    instance Dep x y => Dep (Just x) (Maybe y)
>> |  >
>> |  > Is this supposed to work? I get "The coverage condition fails"
>> |  errors.
>> |  >
>> |  > For simple cases like
>> |  >
>> |  >    instance Dep True Bool
>> |  >
>> |  > etc. it seems to work fine.
>> |  >
>> |  > Thanks and cheers,
>> |  >
>> |  >    Gabor
>> |  > _______________________________________________
>> |  > ghc-devs mailing list
>> |  > ghc-devs at haskell.org
>> |  > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>> |
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>
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