Re: [Haskell-cafe] MPTCs and extended defaulting

22 Sep 2016


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