[Haskell-cafe] Trying to return a map from State monad

[michael rice]

OK. Again, not sure what going on here. Pattern looks OK to me.
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),all@(w1:words)) <- get              if (Prelude.null all)                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) [w1] mp, (pfx2,w1), words)                  buildMap
=============================
*Main> :r[1 of 1] Compiling Main             ( markov3.hs, interpreted )Ok, modules loaded: Main.*Main> fst $ runState buildMap (singleton ("\n","\n") [], ("\n","\n"), ["I","am","lost."])fromList *** Exception: Pattern match failure in do expression at markov3.hs:13:14-44

--- On Thu, 5/19/11, michael rice <nowgate at yahoo.com> wrote:

From: michael rice <nowgate at yahoo.com>
Subject: Re: [Haskell-cafe] Trying to return a map from State monad
To: "Thedward Blevins" <thedward at barsoom.net>
Cc: haskell-cafe at haskell.org
Date: Thursday, May 19, 2011, 12:41 PM

Ok, I see I left out the "State" word.
Should be: type GeneratorState = State (Map Prefix [String],Prefix,[String])
Thanks,
Michael

--- On Thu, 5/19/11, Thedward Blevins <thedward at barsoom.net> wrote:

From: Thedward Blevins <thedward at barsoom.net>
Subject: Re: [Haskell-cafe] Trying to return a map from State monad
To: "michael rice" <nowgate at yahoo.com>
Cc: haskell-cafe at haskell.org
Date: Thursday, May 19, 2011, 12:22 PM

On Thu, May 19, 2011 at 11:03, michael rice <nowgate at yahoo.com> wrote:
> type GeneratorState = (Map Prefix [String],Prefix,[String])

> buildMap :: GeneratorState (Map Prefix [String])

You are trying to use a type alias (GeneratorState) as a type constructor.

There may be other problems, but that leaps out.

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