Re: [Haskell-cafe] How to define a common return and bind?

10 Apr 2009

...
Now I'm wondering if the derive_* functions can be overloaded using
something like this. Note that the following doesn't typecheck:
----------------------------------------
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
----------------------------------------
class Iso m n | m -> n, n -> m where
    close :: forall a. m a -> n a
    open  :: forall a. n a -> m a
deriveReturn :: (Monad m, Monad n, Iso m n) => a -> n a
deriveReturn = close . return
deriveBind :: (Monad m, Iso m n) => n a -> (a -> n b) -> n 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 m (T1 m) where
    close = T1
    open  = unT1
instance Monad m => Monad (T1 m) where
    return = deriveReturn
    (>>=)  = deriveBind
----------------------------------------
Hi, I changed a line, It type checks.
But I can't explain why your version does not type check.

--- iso_orig.hs 2009-04-10 17:56:12.000000000 +0900
+++ iso.hs      2009-04-10 17:56:36.000000000 +0900
@@ -5,7 +5,7 @@

  ----------------------------------------

-class Iso m n | m -> n, n -> m where
+class Iso m n | n -> m where
      close :: forall a. m a -> n a
      open  :: forall a. n a -> m a

Thanks,
Hashimoto

    

Re: [Haskell-cafe] How to define a common return and bind?

Yusaku Hashimoto