Good Haskell coding style of if/else control block?

Question

I\'m learning Haskell in the hope that it will help me get closer to functional programming. Previously, I\'ve mostly used languages with C-like syntax, like C, Java, and D.

Answer 1

Haskell style is functional, not imperative! Rather than "do this then that," think about combining functions and describing what your program will do, not how.

In the game, your program asks the user for a guess. A correct guess is a winner. Otherwise, the user tries again. The game continues until the user guesses correctly, so we write that:

main = untilM (isCorrect 42) (read `liftM` getLine)

This uses a combinator that repeatedly runs an action (getLine pulls a line of input and read converts that string to an integer in this case) and checks its result:

untilM :: Monad m => (a -> m Bool) -> m a -> m ()
untilM p a = do
  x <- a
  done <- p x
  if done
    then return ()
    else untilM p a

The predicate (partially applied in main) checks the guess against the correct value and responds accordingly:

isCorrect :: Int -> Int -> IO Bool
isCorrect num guess =
  case compare num guess of
    EQ -> putStrLn "You Win!"  >> return True
    LT -> putStrLn "Too high!" >> return False
    GT -> putStrLn "Too low!"  >> return False

The action to be run until the player guesses correctly is

read `liftM` getLine

Why not keep it simple and just compose the two functions?

*Main> :type read . getLine

:1:7:
    Couldn't match expected type `a -> String'
           against inferred type `IO String'
    In the second argument of `(.)', namely `getLine'
    In the expression: read . getLine

The type of getLine is IO String, but read wants a pure String.

The function liftM from Control.Monad takes a pure function and “lifts” it into a monad. The type of the expression tells us a great deal about what it does:

*Main> :type read `liftM` getLine
read `liftM` getLine :: (Read a) => IO a

It's an I/O action that when run gives us back a value converted with read, an Int in our case. Recall that readLine is an I/O action that yields String values, so you can think of liftM as allowing us to apply read “inside” the IO monad.

Sample game:

1
Too low!
100
Too high!
42
You Win!