Haskell: words count from a getLine without the "do" notation

Question

How do I write the following function without using the "do" notation?

wordsCount =  do 
    putStr "Write a line and press [enter]:\n"
    s <- getLine
    putStr $ show $ length . words $ s
    putChar '\n'

Answer 1

Instead of using do, you can use >> and >>=:

wordsCount = putStr "Write a line and press [enter]:\n" >> getLine >>= putStr . show . length . words >> putChar '\n'

Or making it easier to read:

wordsCount = putStr "Write a line and press [enter]:\n" >>
    getLine >>=
    putStr . show . length . words >>
    putChar '\n'

A more straight-forward translation would be:

wordsCount = putStr "Write a line and press [enter]:\n" >>
    getLine >>=
    \s -> (putStr $ show $ length $ words s) >>
    putChar '\n'

Basically the compiler converts such do-notation blocks to its monadic equivalent (using only >> and >>=). do is only syntactical sugar such that one does not have to write >>= each time and/or mange variables.

Additional notes:

• as @ChadGilbert said in his comment, the parenthesis should be wrapped around the function, excluding the \s -> such that the s can be used later on in the program, for example:

  -- This is not an equivalent program
  wordsCount = putStr "Write a line and press [enter]:\n" >>
      getLine >>=
      \s -> (putStr $ show $ length $ words s) >>
      putChar '\n' >>
      putStrLn s -- repeat s
  

• Instead of using putStr and putChar, you can use putStrLn. For example:

  wordsCount = putStr "Write a line and press [enter]:\n" >>
      getLine >>=
      putStrLn . show . length . words