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

================================================

import Control.Monad.State

import 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))

==============================================

[michael@hostname ~]$ ghc --make markov4.hs

[1 of 1] Compiling Main ( markov4.hs, markov4.o )

Linking markov4 ...

[michael@hostname ~]$ echo "I am lost." | ./markov4

(fromList [(("\n","\n"),["I"]),(("\n","I"),["am"]),(("I","am"),["lost."])],("am","lost."),[])

[michael@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@hostname ~]$