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
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@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@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
you need "liberal coverage checking", so UndecidableInstances. Are you doing that? | -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@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@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@haskell.org | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs | | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
No. I'll switch that on and report back. Thanks, Gabor Em sexta-feira, 31 de julho de 2015, Simon Peyton Jones < simonpj@microsoft.com> escreveu:
you need "liberal coverage checking", so UndecidableInstances. Are you doing that?
| -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org <javascript:;>] 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@gmail.com <javascript:;>> 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@haskell.org <javascript:;> | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs | | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org <javascript:;> | http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
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@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@microsoft.com> escreveu:
you need "liberal coverage checking", so UndecidableInstances. Are you doing that?
| -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@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@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@haskell.org | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs | | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
With *UndecidableInstances* switchd on it seems to work :-) Thanks for the help! Gabor Em sexta-feira, 31 de julho de 2015, Gabor Greif <ggreif@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@microsoft.com <javascript:_e(%7B%7D,'cvml','simonpj@microsoft.com');>> escreveu:
you need "liberal coverage checking", so UndecidableInstances. Are you doing that?
| -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@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@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@haskell.org | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs | | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
participants (4)
-
Gabor Greif -
Iavor Diatchki -
Richard Eisenberg -
Simon Peyton Jones