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@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@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@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