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.

I have a little question about the coding style of an if/else control block used by the tutorial on Wikibooks. The code looks like the following:

doGuessing num = do
   putStrLn "Enter your guess:"
   guess <- getLine
   if (read guess) < num
     then do putStrLn "Too low!"
             doGuessing num
     else if (read guess) > num
            then do putStrLn "Too high!"
                    doGuessing num
            else do putStrLn "You Win!"

It makes me confused, because this coding style totally violates the recommended style in C-like languages, where we should indent if, else if, and else at the same column.

I know it just does not work in Haskell, because it would be a parse error if I indented else at the same column as if.

But what about the following style? I think it is much more clear than the above one. But since the above is used by Wikibooks and Yet Another Haskell Tutorial, which is marked “best tutorial available online” at the official Haskell website, I’m not sure whether this coding style is a convention in Haskell programs.

doGuessing num = do
    putStrLn "Enter your guess:"
    guess <- getLine
    if (read guess) < num then
        do 
            putStrLn "Too low!"
            doGuessing num
        else if (read guess) > num then do 
            putStrLn "Too high!"
            doGuessing num
        else do 
            putStrLn "You Win!"

So, I’m curious about which coding style is used more often—or is there another coding style for this piece of code?

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

<interactive>: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!