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Gabor Greif

31 Jul 2015 31 Jul '15

9:54 a.m.

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

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Richard Eisenberg

31 Jul 31 Jul

11:10 a.m.

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 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

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Simon Peyton Jones

11:29 a.m.

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 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

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Gabor Greif

12:56 p.m.

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 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

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Iavor Diatchki

1:01 p.m.

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 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 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

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Gabor Greif

3:41 p.m.

With *UndecidableInstances* switchd on it seems to work :-) Thanks for the help! Gabor Em sexta-feira, 31 de julho de 2015, Gabor Greif 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 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

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Gabor Greif
Iavor Diatchki
Richard Eisenberg
Simon Peyton Jones