That works, but I don't understand how I can print the map mp on one line but
then when I try to extract a value out of it it's no longer a map but a triple.
How can mp be a map on one line but not on another?
Michael
=============
[1 of 1] Compiling Main ( markov4.hs, markov4.o )Linking markov4
...[michael@hostname ~]$ echo "I am lost." | ./markov4(fromList
[(("\n","\n"),["I"]),(("\n","I"),["am"]),(("I","am"),["lost."])],("am","lost."),[])["I"][michael@hostname
~]$ ghciGHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for helpLoading
package ghc-prim ... linking ... done.Loading package integer-gmp ... linking
... done.Loading package base ... linking ... done.Loading package ffi-1.0 ...
linking ... done.Prelude> :m + Data.MapPrelude Data.Map> let mp = fromList
[(1,'a'),(2,'b'),(3,'c')]Loading package array-0.3.0.1 ... linking ...
done.Loading package containers-0.3.0.0 ... linking ... done.Prelude Data.Map>
:t mpmp :: Map Integer CharPrelude Data.Map> mpfromList
[(1,'a'),(2,'b'),(3,'c')]Prelude Data.Map> mp ! 2'b'Prelude Data.Map>
--- On Fri, 5/20/11, Arlen Cuss <[email protected]> wrote:
From: Arlen Cuss <[email protected]>
Subject: Re: [Haskell-cafe] Can't access map value with key.
To: "michael rice" <[email protected]>
Cc: [email protected]
Date: Friday, May 20, 2011, 11:36 PM
On Fri, 2011-05-20 at 19:04 -0700, michael rice wrote:
> markov4.hs:35:27: Couldn't match expected type `Map k a'
> against inferred type `(Map Prefix [String], Prefix, [String])'
ghc seems to believe `mp' here is not just a map, but the entire state
(which is what execState is meant to give you).
> type GeneratorState = State ((Map Prefix [String]),Prefix,[String])
The state is a triple, so you need to match to get the real map out:
> let (themap,_,_) = mp
> putStrLn $ show $ themap ! (non_word,non_word)
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