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.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))
==============================================
[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 `($)', namely
`show (mp ! (non_word, non_word))'[michael@hostname ~]$
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
