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README
frisby
Linear time composable parser for PEG grammars.
frisby is a parser library that can parse arbitrary PEG grammars in linear time. Unlike other parsers of PEG grammars, frisby need not be supplied with all possible rules up front, allowing composition of smaller parsers.
PEG parsers are never ambiguous and allow infinite lookahead with no backtracking penalty. Since PEG parsers can look ahead arbitrarily, they can easily express rules such as the maximal munch rule used in lexers, meaning no separate lexer is needed.
In addition to many standard combinators, frisby provides routines to translate standard regex syntax into frisby parsers.
PEG based parsers have a number of advantages over other parsing strategies:
- PEG parsers are never ambiguous
- PEG is a generalization of regexes, they can be though of as extended regexes with recursion, predicates, and ordered choice
- you never need a separate lexing pass with PEG parsers, since you have arbitrary lookahead there is no need to break the stream into tokens to allow the limited LALR or LL lookahead to work.
- things like the maximal munch and minimal munch rules are trivial to specify with PEGs, yet tricky with other parsers
- since you have ordered choice, things like the if then else ambiguity are nonexistent.
- parsers are very very fast, guaranteeing time linear in the size of the input, at the cost of greater memory consumption
- the ability to make local choices about whether to accept something lets you write parsers that deal gracefully with errors very easy to write, no more uninformative parse error messages
- PEG parsers can be fully lazy, only as much of the input is read as is needed to satisfy the demand on the output, and once the output has been processed, the memory is immediately reclaimed since a PEG parser never backtracks
- PEG parsers can deal with infinite input, acting in a streaming manner
- PEG parsers support predicates, letting you decide what rules to follow based on whether other rules apply, so you can have rules that match only if another rule does not match, or a rule that matches only if two other rules both match the same input.
- Traditionally, PEG parsers have suffered from two major flaws:
A global table of all productions must be generated or written by hand, disallowing composable parsers implemented as libraries and in general requiring the use of a parser generator tool like pappy
Although memory consumption is linear in the size of the input, the constant factor is very large.
frisby attempts to address both these concerns.
frisby parsers achieve composability by having a compilation pass, recursive parsers are specified using the recursive do notation 'mdo' which builds up a description of your parser where the recursive calls for which memoized entries must be made are explicit. then runPeg takes this description and compiles it into a form that can be applied, during this compilation step it examines your composed parser, and collects the global table of rules needed for a packrat parser to work.
Memory consumption is much less of an issue on modern machines; tests show it is not a major concern, however frisby uses a couple of techniques for reducing the impact. First it attempts to create parsers that are as lazy as possible -- this means that no more of the file is read into memory than is needed, and more importantly, memory used by the parser can be reclaimed as you process its output.
frisby also attempts to optimize your parser, using specialized strategies when allowed to reduce the number of entries in your memoization tables.
frisby attempts to be lazy in reading the results of parsers, parsers tend to work via sending out 'feeler' predicates to get an idea of what the rest of the file looks like before deciding what pass to take, frisby attempts to optimize these feeler predicates via extra lazyness such that they do not cause the actual computation of the results, but rather just compute enough to determine whether a predicate would have succeeded or not.
(It is interesting to note that the memory efficiency of frisby depends vitally on being as lazy as possible, in contrast to traditional thoughts when it comes to memory consumption)
frisby is a work in progress, it has a darcs repo at http://repetae.net/repos/frisby which may be browsed at http://repetae.net/dw/darcsweb.cgi?r=frisby;a=summary
And its homepage is at http://repetae.net/computer/frisby
to learn more about PEG parsers, see this paper http://pdos.csail.mit.edu/~baford/packrat/popl04 and Bryan Ford's packrat parsing page http://pdos.csail.mit.edu/~baford/packrat/