On 9/15/06, Vivian McPhail <[EMAIL PROTECTED]> wrote:
class Forkable a where
fork :: String -> a -> a -> a
...
{-
instance (Monad m, Forkable (m a), Forkable b) => Forkable (m a -> b) where
fork n a1 a2 a = do
a' <- a
fork n (a1 $ return a') (a2 $ return a')
-}
Let's do manual type checking.
First, fork :: Forkable a => String -> a -> a -> a
So for Forkable (m a -> b)
fork :: String -> (m a -> b) -> (m a -> b) -> m a -> b
Then
fork n a1 a2 a :: b
But you define it as
fork n a1 a2 a = do {...}
So it should be of type Monad t => t a, not just any `b'.
Instead, you can define
instance (Monad m, Forkable (m b)) => Forkable (m a -> m b) where
...
Note that to compile it you also need -fallow-undecidable-instances
and -fallow-overlapping-instances.
--
Tolik
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