On Wed, Jul 20, 2011 at 8:41 PM, Thomas
eval_begin :: Continuation a => Int -> a -> Int eval_begin n k = eval n (if (n < 0) then k else (BeginCont k (n - 1)))
Although 'eval n' is polymorphic, the if expression needs to have just one type, either 'a' or 'BeginCont a', and they can't be unified. The solution is pretty simple, though, since eval's return type doesn't mention 'a' at all: eval_begin :: Continuation a => Int -> a -> Int eval_begin n k = if (n < 0) then eval n k else eval n (BeginCont k (n - 1)) Note that 'eval n' is repeated. If you don't to repeat it on your real world code you may give it an explicit name. However you'll need to provide a type signature from GHC 7.0 onwards: eval_begin :: Continuation a => Int -> a -> Int eval_begin n k = let eval' :: Continuation a => a -> Int eval' = eval n in if (n < 0) then eval' k else eval' (BeginCont k (n - 1)) HTH, =) -- Felipe.