[Haskell-cafe] MPTCs and extended defaulting

[Tom Murphy]

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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