On Fri, Apr 10, 2009 at 11:15 AM, Yusaku Hashimoto <[email protected]> wrote:
> Hi, I changed a line, It type checks.
> But I can't explain why your version does not type check.
Thanks, I can't explain it either because I don't completely
understand functional dependencies.
So I rewrote it using type families which I find much easier to understand:
----------------------------------------
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleContexts #-}
module Iso where
class Iso m where
type Inner m :: * -> *
close :: forall a. Inner m a -> m a
open :: forall a. m a -> Inner m a
deriveReturn :: (Monad (Inner m), Iso m) => a -> m a
deriveReturn = close . return
deriveBind :: (Monad (Inner m), Iso m) => m a -> (a -> m b) -> m b
deriveBind m k = close $ open m >>= open . k
----------------------------------------
newtype T1 m a = T1 { unT1 :: A1 m a }
type A1 m a = m a
instance Iso (T1 m) where
type Inner (T1 m) = m
close = T1
open = unT1
instance Monad m => Monad (T1 m) where
return = deriveReturn
(>>=) = deriveBind
----------------------------------------
You can also get a Monad instance "for free" if you aren't afraid of
UndecidableInstances:
----------------------------------------
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
instance (Monad (Inner m), Iso m) => Monad m where
return = deriveReturn
(>>=) = deriveBind
----------------------------------------
This seems useful. Does this exists somewhere on hackage?
regards,
Bas
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