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

[michael rice]

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 <aditya.siram at gmail.com> wrote:

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