Re: [Haskell-cafe] MPTCs and extended defaulting

22 Sep 2016


      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  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