Question

How can we print current state value, for debugging purposes? E.g., in code from concrete-example-1 of http://www.haskell.org/haskellwiki/State_Monad , how can we print the current state value after each input character is read?

module StateGame where

import Control.Monad.State

type GameValue = Int
type GameState = (Bool, Int)

playGame :: String -> State GameState GameValue
playGame []     = do
    (_, score) <- get
    return score

playGame (x:xs) = do
    (on, score) <- get
    case x of
       'a' | on -> put (on, score + 1)
       'b' | on -> put (on, score - 1)
       'c'      -> put (not on, score)
       _        -> put (on, score)
    playGame xs

startState = (False, 0)

main = print $ evalState (playGame "abcaaacbbcabbab") startState

Answer 1

For quick and dirty debugging, use trace from Debug.Trace. I often find it useful to flip the argument order and define

infixl 0 `debug`

debug :: a -> String -> a
debug = flip trace

Then you can tack the debugging to the end of the line and it’s easier to comment out (and at the end remove).

For more principled logging, combine the State with a Writer and tell the logging messages or use StateT s IO if you directly want to log to a file or stderr.