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 wrote: From: michael rice Subject: Re: [Haskell-cafe] Trying to return a map from State monad To: "Thedward Blevins" 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 wrote: From: Thedward Blevins Subject: Re: [Haskell-cafe] Trying to return a map from State monad To: "michael rice" Cc: haskell-cafe@haskell.org Date: Thursday, May 19, 2011, 12:22 PM On Thu, May 19, 2011 at 11:03, michael rice 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. -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

A laugh a minute. I though of that and tried it but got the same response (forgetting to :r my source) before rerunning. Time for a break. Thanks! Michael  --- On Thu, 5/19/11, aditya siram wrote: From: aditya siram Subject: Re: [Haskell-cafe] Trying to return a map from State monad To: "michael rice" Cc: "Thedward Blevins" , haskell-cafe@haskell.org Date: Thursday, May 19, 2011, 4:57 PM The problem is that the "all@(w1:words)" pattern-match fails when "all" is empty. The quick and dirty fix is:     import Control.Monad.State     import Data.Map     import Debug.Trace         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) <- 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) [head all] mp, (pfx2,head all), tail all)                       buildMap *Main>  fst $ runState buildMap (singleton ("\n","\n") [], ("\n","\n"), ["I","am","lost."]) fromList [(("\n","\n"),["I"]),(("\n","I"),["am"]),(("I","am"),["lost."]),(("am","lost."),["\n"])] A better one would be to write a helper function that correctly pattern matched on the list. -deech On Thu, May 19, 2011 at 3:30 PM, michael rice wrote: 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 wrote: From: michael rice Subject: Re: [Haskell-cafe] Trying to return a map from State monad To: "Thedward Blevins" 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 wrote: From: Thedward Blevins Subject: Re: [Haskell-cafe] Trying to return a map from State monad To: "michael rice" Cc: haskell-cafe@haskell.org Date: Thursday, May 19, 2011, 12:22 PM On Thu, May 19, 2011 at 11:03, michael rice 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. -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe