Ben Franksen wrote:
I was refering to the 1996 paper titled "Deterministic, Error-Correcting Combinator Parsers", where the 'fail' parser needs extra arguments. Maybe I am dense, but I can't see how to avoid the arguments w/o constraining the type of the result (which would mean the parsers can't even be instances of Applicative).
The trick is to not constrain the result type: pFail should be polymorphic in its result type. Suppose you use the list of successes parser:
type Parser s a = [s] -> [(a, [s])]
Then pFail may be defined as:
pFail :: Parser s a pFail = const []
i.e. polymorphic in output (and input too), because [] :: [a] is polymorphic in its element type. I'm pretty sure most if not all modern parse libraries contain such a function. HTH, Martijn.