On 3/28/06, Neil Mitchell
This feels like a situation Parsec users would find themselves in all the time. When you have a bunch of parsers in a 'choice', does the start of the input stream linger until the last parser is executed?
No, as soon as one token is accepted from any parser, that input is decided upon, and it will never go back. If you want that behaviour you have to wrap the particular parser in try, which does give the backtracking (and space leak)
I personally find this behaviour terribly confusing. It makes writing the parser highly unmodular. It forces me to know exactly what a certain parser recognizes to know whether I need to wrap a 'try' around it when I compose it in parallel with another parser. Which is why I much prefer to use parsing libraries based on Koen Claessen's technique which performs all parses at the same time. It works breadth-first on the parse forest (the set of parse trees). Text.ParserCombinators.ReadP is one example which uses this technique. Cheers, /Josef