[Haskell-cafe] Parsec question: how to access parser state

Andrew Pimlott andrew at pimlott.net
Sun Mar 20 20:58:40 EST 2005

On Sun, Mar 20, 2005 at 03:32:38PM -0500, Dimitry Golubovsky wrote:
> type TParser a = GenParser Token (FiniteMap String Declaration) a
> The FiniteMap (which is the user state) is expected to be updated during 
> parsing whus building some internal lookup table.
> and in one of the parsing functions I want to call
> show getState
> assuming it will apply show to the FiniteMap and return some string of 
> characters.
> During compliation, I get:
>     No instance for (Show (GenParser tok st st))
>       arising from use of `show' at (file location)
>     In the first argument of `xxxx', namely `(show getState)'

This is a good clue--you don't want to show a parser, you want to show
the state.  Look at the type of getState and think monadic thoughts.
There is in fact an example of its use in the Parsec documentation.


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