[Haskell-cafe] Can't access map value with key.

[michael rice]

What am I doing wrong in the last line of main? If I comment out the last line of main, my program prints the map (mp) that is created. If I leave it in, the program fails. The last line should print the list ["I"].
Michael
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import Control.Monad.Stateimport Data.Map
type Prefix = (String,String)type GeneratorState = State ((Map Prefix [String]),Prefix,[String])

non_word = "\n"
f key new old = new ++ old 
buildMap :: GeneratorState (Map Prefix [String])buildMap = do (mp,(pfx1,pfx2),words) <- get              if (Prelude.null words)                then  {- No more words. Return final map (adding non_word for prefix). -}                  return (insertWithKey' f (pfx1,pfx2) [non_word] mp)                else do {- Add word to map at prefix & continue. -}                  put (insertWithKey' f (pfx1,pfx2) [head words] mp, (pfx2,(head words)), tail words)                  buildMap
{-*Main> execState buildMap (singleton ("\n","\n") [], ("\n","\n"), ["I","am","lost."])(fromList [(("\n","\n"),["I"]),(("\n","I"),["am"]),(("I","am"),["lost."])],("am","lost."),[])*Main> -}

nwords = 1
main = do contents <- getContents          let mp = execState buildMap (singleton (non_word,non_word) [], (non_word,non_word), words contents)          putStrLn $ show mp          putStrLn $ show (mp ! (non_word,non_word)) 


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[michael at hostname ~]$ ghc --make markov4.hs[1 of 1] Compiling Main             ( markov4.hs, markov4.o )Linking markov4 ...[michael at hostname ~]$ echo "I am lost." | ./markov4(fromList [(("\n","\n"),["I"]),(("\n","I"),["am"]),(("I","am"),["lost."])],("am","lost."),[])[michael at hostname ~]$ ghc --make markov4.hs[1 of 1] Compiling Main             ( markov4.hs, markov4.o )
markov4.hs:35:27:    Couldn't match expected type `Map k a'           against inferred type `(Map Prefix [String], Prefix, [String])'    In the first argument of `(!)', namely `mp'    In the first argument of `show', namely        `(mp ! (non_word, non_word))'    In the second argument of `($)', namely        `show (mp ! (non_word, non_word))'[michael at hostname ~]$ 
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