newbie: running a state transformer in context of a state rea der

21 Feb 2001 07:00:39 GMT

Tue, 20 Feb 2001 17:52:33 -0800, Konst Sushenko <konsu@microsoft.com> pisze:

> lookahead p = do { s <- fetch
>                  ; lift (evalState p s)
>                  }
> 
> is typed as
> 
> lookahead:: State MyState Maybe a -> State MyState Maybe (a,MyState)
> 
> but i need
> 
> lookahead:: State MyState Maybe a -> State MyState Maybe a

myEvalState = liftM fst yourEvalState

Andy Gill's monadic modules provide evalState as a wrapper for runState,
which throws away the state component returned.

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