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

20 May 2011

      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 <nowgate@yahoo.com> 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 <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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Re: [Haskell-cafe] Trying to return a map from State monad

aditya siram