OK. Again, not sure what going on here. Pattern looks OK to me.

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),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@yahoo.com> wrote:

From: michael rice <nowgate@yahoo.com>
Subject: Re: [Haskell-cafe] Trying to return a map from State monad
To: "Thedward Blevins" <thedward@barsoom.net>
Cc: haskell-cafe@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@barsoom.net> wrote:

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

On Thu, May 19, 2011 at 11:03, michael rice <nowgate@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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