Fundep question

[Iavor Diatchki]

Yeah, this should work with UndecidableInstances.  The reason you need that
extension is that if we consider the context, then there are some
pathological examples that can make GHC's instance checking algorithm go
forever.

On Fri, Jul 31, 2015 at 9:56 AM, Gabor Greif <ggreif at gmail.com> wrote:

> 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> 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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