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

Question

I am trying to read a file into a function to calculate the frequencies of characters in a file. So I am trying the following:

charCount :: String -> [(Char, Int)]
charCount input = M.toList $ M.fromListWith (+) [(c, 1) | c <- input]

calculate :: FilePath -> [(Char, Int)]
calculate fp = do
  c <- readFile fp
  charCount c

But I am getting the following error:

FileWriter.hs:13:8: Couldn't match expected type ‘[String]’ …
                with actual type ‘IO String’
    In a stmt of a 'do' block: c <- readFile fp
    In the expression:
      do { c <- readFile fp;
           charCount c }
Compilation failed.

Answer 1

Since calculate calls the readFile function which returns a value wrapped in the IO monad the function calculate must return a IO value too, and the result of the call to charCount (a pure computation) has to be return ed in order to wrap the [(Char, Int)] into a monad.

The next example worked in ghc 7.10.1 :

import qualified Data.Map as M

charCount :: String -> [(Char, Int)]
charCount input = M.toList $ M.fromListWith (+) [(c, 1) | c <- input]

calculate :: FilePath -> IO [(Char, Int)]
calculate fp =
    readFile fp >>= \c ->
    return (charCount c)