[Haskell-cafe] MPTCs and extended defaulting

[Tom Murphy]

Ah, looks like that's unrelated to the original issue, though -- thanks!

Tom


On Fri, Sep 23, 2016 at 9:15 AM, Tom Murphy <amindfv at gmail.com> wrote:

> With GHC 8.0.1 I get a type error:
>
> error:
>     • Could not deduce (Foo x a0)
>       from the context: Foo x a
>         bound by the type signature for:
>                    foo :: Foo x a => x -> AB
>
>
> On Fri, Sep 23, 2016 at 8:56 AM, Christopher Allen <cma at bitemyapp.com>
> wrote:
>
>> {-# LANGUAGE DataKinds #-}
>> {-# LANGUAGE ExtendedDefaultRules #-}
>> {-# LANGUAGE FlexibleInstances #-}
>> {-# LANGUAGE KindSignatures #-}
>> {-# LANGUAGE MultiParamTypeClasses #-}
>> {-# LANGUAGE TypeFamilies #-}
>>
>> import GHC.TypeLits
>>
>> data AB = A | B Double deriving (Show)
>>
>> class Foo x (a :: [Symbol]) where
>>    foo :: x -> AB
>>
>> instance (b ~ Double) => Foo b a where
>>    foo = B
>>
>> main = print $ foo 5
>>
>> On Thu, Sep 22, 2016 at 6:39 PM, Tom Murphy <amindfv at gmail.com> wrote:
>> > Bump -- is this feasible? It would be extremely helpful for an EDSL of
>> mine.
>> >
>> > Tom
>> >
>> > On Tue, Aug 30, 2016 at 5:23 AM, Tom Murphy <amindfv at gmail.com> wrote:
>> >>
>> >> I've got a case in a library I'm working on where having
>> >> -XExtendedDefaultRules with MPTCs would be very, very helpful. Is it
>> >> possible?
>> >>
>> >> I.e., we can now write:
>> >>
>> >> ```
>> >> {-# LANGUAGE ExtendedDefaultRules #-}
>> >>
>> >> data AB = A | B Double deriving (Show)
>> >>
>> >> class Foo x where
>> >>    foo :: x -> AB
>> >>
>> >> instance Foo Double where
>> >>    foo = B
>> >>
>> >> main = print $ foo 5
>> >> ```
>> >>
>> >> And -XExtendedDefaultRules makes sure we don't need to write "5 ::
>> Double"
>> >>
>> >>
>> >> If, though, I want 'Foo' to take another parameter (here, a ::
>> [Symbol]),
>> >> it falls apart:
>> >>
>> >> ```
>> >> {-# LANGUAGE DataKinds #-}
>> >> {-# LANGUAGE ExtendedDefaultRules #-}
>> >> {-# LANGUAGE KindSignatures #-}
>> >> {-# LANGUAGE MultiParamTypeClasses #-}
>> >>
>> >> import GHC.TypeLits
>> >>
>> >> data AB = A | B Double deriving (Show)
>> >>
>> >> class Foo x (a :: [Symbol]) where
>> >>    foo :: x -> AB
>> >>
>> >> instance Foo Double a where
>> >>    foo = B
>> >>
>> >> main = print $ foo 5
>> >> ```
>> >>
>> >> Is there a reason MPTCs can't support ExtendedDefaultRules?
>> >>
>> >> Thanks!
>> >> Tom
>> >>
>> >>
>> >>
>> >>
>> >
>> >
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>>
>>
>>
>> --
>> Chris Allen
>> Currently working on http://haskellbook.com
>>
>
>
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