[Haskell-cafe] Can't access map value with key.

Arlen Cuss celtic at sairyx.org
Sat May 21 05:36:53 CEST 2011

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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