On Tue, 08 Dec 2015 23:29:22 +0100, martin
data Tree a = B a (Tree a) (Tree a) | L a Char deriving Show
get (B a _ _) = a get (L a _) = a
tcmp = compare `on` get
build :: [Tree Int] -> [Tree Int] build (t:[]) = [t] -- build t = let xs = sortBy (compare `on` get) t -- < -- build t = let xs = sortBy tcmp t in build (merge (xs!!0) (xs!!1) : drop 2 xs)
The commented line -- < -- does not work, though I am just replacing equals with equals. I get
No instance for (Ord b0) arising from a use of ‘compare’ The type variable ‘b0’ is ambiguous Relevant bindings include tcmp :: Tree b0 -> Tree b0 -> Ordering : Why is that so?
If you do not use the function tcmp, the compiler can not deduce the type of it. When you add a type like tcmp :: Ord a => Tree a -> Tree a -> Ordering , the compiler has enough information to compile the program. Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --