Re: [Haskell-cafe] Why is Haskell flagging this?

On 17 December 2010 18:04, michael rice wrote:

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f :: [Int] -> IO [Int] f lst = do return lst

main = do let lst = f [1,2,3,4,5] fmap (+1) lst

The problem is that you are applying fmap to a type IO a. fmap (+1) (return [1,2,3]) But to achieve the behaviour you expect, you need another fmap: fmap (fmap (+1)) (return [1,2,3])