On 08/05/07, Matthew Sackman
:t let f r s = let g (fn::forall n . (Num n) => n -> n) = return (fn r, fn s) in (return negate) >>= g in f
Ah, I may have been off the mark earlier. I think the problem is due to the fact that you can't pass higher-order polymorphic functions around. I.e., the following is a classic example of something people expect to work, but doesn't: runST $ ... runST is a rank-2 polymorphic function, and you're attempting to pass it as a parameter to the ($) function, which doesn't work. I think your problem is similar. Here's the module I used to investigate goings on: {-# OPTIONS_GHC -fglasgow-exts #-} import Data.Complex g :: (Num a, Num b, Monad m) => a -> b -> (forall n. Num n => n -> n) -> m (a, b) g r s fn = return (fn r, fn s) f :: Maybe (Int, Complex Float) f = return negate >>= g (4 :: Int) (1 :+ 2) You're attempting to pass the rank-2 polymorphic function "g (4 :: Int) (1 :+ 2)" as a parameter to (>>=), which doesn't work. General point: couldn't GHC's error reporting be improved at times like these? -- -David House, dmhouse@gmail.com