Couldn't match expected type ‘IO [String]’ with actual type ‘[String]’

Question

I have these two code snippets, which I'd guess do the same thing, but they don't. Why is that?

This one works fine:

fdup :: String -> IO ()
fdup filename = do
        h <- openFile filename ReadMode
        c <- hGetContents h
        putStr $ unlines $ parse $ lines c
        hClose h

This one returns an error Couldn't match expected type ‘IO [String]’ with actual type ‘[String]’:

fdup' :: String -> IO ()
fdup' filename = do
        h <- openFile filename ReadMode
        c <- hGetContents h
        ls <- lines c
        putStr $ unlines $ parse $ ls
        hClose h

parse :: [String] -> [String]

What is the difference between them?

Answer 1

As Willem Van Onsem explained, you don't need <- in that specific place because lines c is just a list of strings, and not an IO computation. If you want to give it a name, you can use a let-binding instead:

fdup' :: String -> IO ()
fdup' filename = do
        h <- openFile filename ReadMode
        c <- hGetContents h
        let ls = lines c
        putStr $ unlines $ parse $ ls
        hClose h