Haskell Quiz/Morse Code/Solution Dolio
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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 a -> Parser a [a]
many p = many1 p `mplus` return []
many1 :: Parser a a -> Parser a [a]
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 Char
morseLetter = msum $ zipWith (\c l -> string c >> return l) morse ['a'..'z']
main = interact $ unlines . concatMap (evalStateT (total $ many morseLetter)) . words