Haskell Quiz/Morse Code/Solution Dolio

This solution uses a naive implementation of monadic parser combinators, simply using the state transform of the list monad. For more information on how all this works, a google search for 'monadic parser combinators' should be more than sufficient.

import Data.List
import Control.Monad
import Control.Monad.State

-- The classic parser monad, with input token type a, and output type b
type Parser a b = StateT [a] [] b

morse = [ ".-", "-...", "-.-.", "-..", ".", "..-.", "--.", "....", "..", ".---"
        , "-.-", ".-..", "--", "-.", "---", ".--.", "--.-", ".-.", "...", "-"
        , "..-", "...-", ".--", "-..-", "-.--", "--.." ]

token :: Parser a a
token = do (a:as) <- get ; put as ; return a

satisfy :: (a -> Bool) -> Parser a ()
satisfy p = token >>= guard . p

many :: Parser a b -> Parser a [b]
many p = many1 p `mplus` return []

many1 :: Parser a b -> Parser a [b]
many1 p = liftM2 (:) p (many p)

total :: Parser a b -> Parser a b
total p = p >>= \b -> get >>= guard . null >> return b

string :: String -> Parser Char ()
string [] = return ()
string (x:xs) = satisfy (==x) >> string xs

morseLetter :: Parser Char (String, Char)
morseLetter = msum $ zipWith (\c l -> string c >> return (c,l)) morse ['a'..'z']

main = interact $ unlines
                    . concatMap (map showMorse
                                    . evalStateT (total $ many morseLetter))
                    . words

intercalate :: [a] -> [[a]] -> [a]
intercalate l = concat . intersperse l

showMorse :: [(String, Char)] -> String
showMorse = (\(m,l) -> pad l ++ intercalate "|" m) . unzip
 where pad l = l ++ replicate (30 - length l) ' '