On Tue, May 06, 2003 at 09:05:46PM +0000, sashan wrote:
I'm trying to write a function that builds a list from user input. This is what I've come up with so far.
--a program that makes a list of numbers from user input makeList :: IO [Int] makeList = do s <- getLine if read s /= 0 then return (read s:makeList) else return []
However it fails with the following error: Couldn't match `[a]' against `IO [Int]' Expected type: [a] Inferred type: IO [Int] In the second argument of `(:)', namely `makeList' In the first argument of `return', namely `((read s) : makeList)'
It's trying to match a list, [a], with type IO [Int]. This causes it to fail. How do I get around this?
This is happening because makeList returns IO [Int], whereas the (:) operator or only defined over [a]. It doesn't work for the same reason that you can't use getLine as a String. This should work: makeList :: IO [Int] makeList = do s <- getLine case read s of 0 -> return [] x -> do xs <- makeList return (x:xs) I changed the if to a case so that (read s) doesn't get called twice. Hope that helps. galen