On Fri, Aug 19, 2011 at 14:06, Anupam Jain
Hi all, Suppose I have a compound data type - data M o = M (String,o) Now, I can define a function that works for ALL M irrespective of o. For example - f :: M o -> M o f (M (s,o)) = M (s++"!", o) I can also use this function in an expression, applying it to different types without problem - p = (m1',m2') where m1 = M ("1", ()) m2 = M ("2", True) m1' = f m1 m2' = f m2 Main*> p (M ("1!",()),M ("2!",True)) However, if I try to parameterise over the function 'f' it does not work! - p f = (m1',m2') where m1 = M ("1", ()) m2 = M ("2", True) m1' = f m1 m2' = f m2 It doesn't even typecheck, producing the error - "Couldn't match expected type 'Bool' with actual type '()'" Is there a particular reason for this? How can I define a function like 'p' within Haskell?
If you write down the type for 'p', you get something like this: p :: (forall a. M a -> M a) -> (M b, M b) That is, the type variable 'a' isn't top level, but is forall'ed in the function you pass. This is called a rank 2 type, and it cannot be inferred automatically by GHC. You have to specify it yourself, and turn on the Rank2Types or RankNTypes extension. Erik