See comments below. On Sun, May 12, 2002, David Feuer wrote:
On Sun, May 12, 2002, Emre Tezel wrote:
Hi all,
I recently bought Simon Thompson's Haskell book. I have been doing the exercises while I read on. There are couple questions that I can not solve. Any help would be greatly appreciated.
I got stuck on this question in Chapter 10.
Define a function composeList which composes a list of functions into a single function. What is the type of composeList?
I naively tried the following but the hugs compiler complained about the inferred type not being generic enough.
composeList :: [(a -> b)] -> (c -> b) composeList [] = id composeList (x:xs) = x . (composeList xs)
<snip>
You can do a lot better in Glasgow Haskell:
data Fun a b = forall c . Comp (c -> b) (Fun a c) | End (a -> b)
compose :: Fun a b -> a -> b --GHC needs this type signature --to compile the program, but --I don't understand why. --Any tips? compose (End f) = f compose (Comp f l) = f . compose l
Well, I figured out why the type signature is necessary (polymorphic recursion), but I don't understand the error message I got from GHC: cc.hs:5: Inferred type is less polymorphic than expected Quantified type variable `c' escapes When checking a pattern that binds f :: c -> b l :: Fun a c In the definition of `compose': compose (Comp f l) = f . (compose l) What makes it think that `c' escapes? This message had me staring at the code the wrong way for quite a while before I decided to add a type signature and see if that gave me any more useful information.
f::Int -> Float f x = fromIntegral x
g::String -> Int g = read
h::Int -> String h x = take x "123456789"
main = do putStrLn "hello!" print $ compose (End (\x -> "Foo!")) 3 print $ compose (Comp f (Comp g (End h))) 4
-- Night. An owl flies o'er rooftops. The moon sheds its soft light upon the trees. David Feuer