Stephen Tetley
Hi Erik
Malcolm Wallace describes a commit combinator in the paper "Partial parsing: combining choice with commitment" which sounds like what you would want. It is implemented for Polyparse rather than Parsec though. From a quick scan of the paper and code, the implementation appears to be built into the Parser type, so it is a primitive rather than a definable combinator.
From the Polyparse homepage (http://www.cs.york.ac.uk/fp/polyparse/):
,---- | If you are familiar with the Parsec library, then the key insight for | using PolyParse is that the two libraries' approach to backtracking | are the duals of each another. In Parsec, you must explicitly add a | try combinator at any location where backtracking might be | necessary. Users often find this a bit of a black art. In PolyParse by | contrast, all parsers are backtracking unless you explicitly add a | commit (or one of its variations). It is easy to tell where to add a | commit point, because you have already parsed enough of a data | structure to know that only one outcome is possible. For instance, if | you are parsing a Haskell value produced by 'show', then as soon as | you have parsed the initial constructor, you know that no other | constructor of that datatype is possible, so you can commit to | returning it. `---- -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com