[Haskell-cafe] Parsers are monadic?

[Malcolm Wallace]

"Claus Reinke" <claus.reinke at talk21.com> wrote:

> (b) i like my combinator grammars to be reversible, so that a single
>     grammar specification can be used for both parsing and
>     unparsing/pretty-printing.  that means i have to define the
>     details myself anyway.

Oh cool - this is something I have wanted for a long time.  Anything
released or otherwise available?

> about the only thing that spoils this nice setup is error handling.
>                ...  so it is easier to  see where it is headed: a
> three-valued logic. 

I wrote a set of monadic combinators (the polyparse library) for exactly
this reason - improving error messages (in HaXml originally).  As
indeed, the basic error structure is a three-valued logic, with success
and two separate kinds of error:
   type EitherE a b = Either (Bool,a) b

The error categories I used were "recoverable failure", to enable
backtracking, and "hard failure" to disallow backtracking when no
alternative good parses were possible.  The combinator 'commit' changes
recoverable failures into hard failures.

I guess you could easily model this setup with three continuations
instead of a monad.

Regards,
    Malcolm