Haskell Compile Question

gistfile1.hs

import Control.Monad

-- Compiles
loadPaths :: FilePath -> IO [Int]
loadPaths filename = do strings <- liftM lines $ readFile filename
                        return (map read strings)


-- Doesn't Compile
loadPaths :: FilePath -> IO [Int]
loadPaths filename = do strings <- liftM lines $ readFile filename
                        map read strings

Do you need an explanation of why the second one doesn't compile?

Yes, also is the use of liftM a good idea?

In a do statement for the monad m every line (exception: let lines; see below) needs to have a type in the monad, i.e. m a for some a.
In the second code, the last line has a type of [Int], which is not in the monad. Using return lifts the value to the monad so that the type is IO [Int].

Using liftM is okey, but could be avoided using let:

loadPaths filename = do content <- readFile filename
                        let strings = lines content
                        return (map read strings)

Thank!