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packrat parser-combinator library for coffeescript / javascript
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README.md

packrattle

this is a simple packrat parser-combinator library for coffeescript / javascript.

an example, from the unit tests:

parser = require("packrattle")

csv = parser.repeat(
  parser.regex(/([^,]*)/).onMatch (m) -> m[0]
  /,/
)

csv.parse("this,is,csv")
=> { ok: true, match: [ "this", "is", "csv" ] }

there's a wiki page on parser-combinators here: http://en.wikipedia.org/wiki/Parser_combinator

but the basic idea is that a "parser" is a function that takes a string and position, processes some chunk of the string, and returns a new position, along with the digested part of the string. in other words, a parser does:

(position) -> (position, result)

on success, or

(position) -> (position, error_message)

on failure.

you can start with a few basic parsers which match a string or regex, and build more complex parsers out of functions that combine them: "a or b", "a then b", "repeat(a)", and so on.

a "packrat" parser just keeps a cache (for the duration of parsing a single string) of the results for each parser at each position, so that if there's a lot of backtracking, it doesn't do the same work over and over again.

basic methods

a few basic parsers just process a chunk of text:

  • parser.string(...) - match exactly this string, and return it

  • parser.regex(...) - match this regex, and return the "match" object (which can be used to extract any groups)

  • parser.end - matches only the end of the string

  • parser.reject - always fails to match

combinators

the real power is in combining the parsers:

  • parser.seq(p1, p2, ...) - match all of p1, p2, ... in sequence; the match result will be an array of all of the non-null match results of p1 and friends

  • parser.optional(p, defaultValue) - match p or return the default value (usually the empty string), succeeding either way

  • parser.check(p) - verify that p matches, but don't advance the parser's position

  • parser.repeat(p, sep) - match p multiple times (often written as "p*"), optionally separated by sep (for example, a comma); the match result will be an array of all the non-null match results, not including the separators

  • parser.times(count, p) - match p exactly count times; the match result will be an array of all the non-null match results

  • parser.foldLeft(args) - see below

each parser has methods on it, also, to allow for combining:

  • onFail(newMessage) - replace the error message for this parser when it fails to match

  • onMatch(function) - transform the result of this parser: the function takes the parser's match result and returns the new result

  • matchIf(function) - if the parser matches, the function gets the parser's match result and returs true/false -- if false, the parser fails to match after all

  • not() - if the parser matches, it fails; if it fails, it matches and returns the empty string

  • or(p) - if the parser matches, it returns normally, but if it fails, p is tried instead

  • skip(p) - check if p matches before trying this parser, and throw p's result away if it matches -- useful for skipping whitespace before a parser

  • then(p) - if this parser matches, try matching p next (just like seq)

  • optional() - make this parser optional, like parser.optional

  • repeat(sep) - just like parser.repeat

  • times(count) - just like parser.times

  • reduce(sep, function) - a simpler variant of foldLeft (see below)

  • drop() - if this parser matches, return null as the match result, which will cause it to be omitted from the result of parser.seq

foldLeft

foldLeft is a slightly more complex/powerful combinator, which matches a sequence of nested parsers with optional separators and combines them as they are parsed. it takes a hash of key/value parameters:

  • first - the parser to match the first occurance (defaults to tail if not supplied)
  • tail - the parser to match all successive occurances
  • sep - optional parser to match the things separating the items (comma, for example); if missing or null, items don't have separators
  • accumulator - a function to transform the first match result into a running accumulator of the parser (defaults to an array containing only the first match result)
  • fold - a function to transform the current accumulator and the new item (and its separator) into a new accumulator: fold(accumulator, sep, item) (defaults to calling accumulator.push(item))

for example, to match a sequence of numbers separated by "+" and add them:

number = parser.regex(/\d+/).onMatch (m) -> parseInt(m[0])
expr = parser.foldLeft(
  tail: number
  sep: parser.string("+")
  accumulator: (n) -> n
  fold: (sum, op, n) -> sum + n
)

this is aliased to "reduce" on Parser, with a simplified interface:

number = parser.regex(/\d+/).onMatch (m) -> parseInt(m[0])
expr = parser.foldLeft(
  tail: number
  sep: parser.string("+")
  accumulator: (n) -> n
  fold: (sum, op, n) -> sum + n
)

implicit conversion

for many functions, an object that isn't a parser will be converted into a parser at runtime, to simplify your code:

  • a string will be converted to parser.string(...)

  • a regex will be converted to parser.regex(...)

  • an array will be converted to parser.seq(...)

  • a function will be called, under the assumption that it returns a parser -- but only when the parser is parsed, allowing for lazy evaluation

executing

to execute a parser, call either:

  • parse(string) - matches as much of the string as it can
  • consume(string) - matches the entire string, or fails

each function returns an object with state:

  • ok - true if the parser succeeded, false if not
  • state - a ParserState object with the current position (see below)
  • match - the match result (if ok is true)
  • message - a string error message (if ok is false)

the ParserState object contains:

  • text - the original string
  • pos - the index within the string that the parser stopped
  • lineno - the current line number of pos, assuming \n divides lines, counting from 0
  • xpos - the position of pos within the current line, counting from 0
  • line() - returns the content of the line around pos (the lineno line)
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