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 .
