Vivian McPhail wrote:
class Forkable a where fork :: String -> a -> a -> a .... What I would like to be able to do is differentiate between Forkable (m a -> b) and Forkable (<function type> -> b).
Have you tried this combination of instances?
instance Forkable (IO a) where ... -- and similarly for all the concrete -- monad types you will use fork with
instance (Forkable a, Forkable b) => Forkable (a -> b) where ...
Alternatively, since the fork function seems to be all about propagating a value (the String), would Control.Monad.Reader serve your purpose?
The value that gets 'forked' is not actually the string, it is the result of a monadic computation.
Regards, Tom
I have tried:
instance Forkable (USM NRef) where...
Which is my Monad The problem lies with
instance (Forkable a, Forkable b) => Forkable (a -> b) where fork n a1 a2 a = fork n (a1 a) (a2 a)
because I need the arg a to be evaluated before it gets passed to a1 and a2. This definition does the right thing when type 'a' is a function type, because it is not a value, but with something like 'm a -> (m a -> m a) -> m a' with Forkable (a -> b) the first arg gets evaluated twice, to be more concrete: With (and golden white) eggs I want the 'eggs' that is passed to 'golden' to be the same as the 'eggs' that is passed to 'white', i.e. -> and1 (golden2 eggs3) (white4 eggs3) and not -> and1 (golden2 eggs3) (white4 eggs5) So to do this I need to be able to recognise the case where the 'a' of (a -> b) is of type 'm a' so that I can evaluate it -- doesn't typecheck 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') Tom suggested that I might be able to use the Reader monad, but I'm not clear as to how I could do this. Cheers, Vivian -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.1.405 / Virus Database: 268.12.5/451 - Release Date: 19/09/2006