Re: [Haskell-cafe] MPTCs and extended defaulting

{-# 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 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 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

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Chris Allen Currently working on http://haskellbook.com