As I have remarked before, treeparse was influenced by Haskell's parsec. One of the features of parsec is the optional infusion of more meaningful error messages into grammars. One could easily imagine adding this functionality to treeparse.
Interesting. I suppose this means that (at least for the cps version of treeparse) a fail must also "return" an error value (potentially just a "Expected lit %c, not found"). Of course 'alt parsers would ignore the error message, and generate its own when all alternatives fail ("No alternatives found"). Hmm. I think if the fail continuation returned an error message, we could create a filt-style parser (for the cps case):
For the monadic version, our old nil-as-fail code would have to be changed I suspect; basically instead of the old type Return = Return parsed remaining actions | nil, we'll need type Return = Return parsed remainig actions | Failed message .
