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  is a library for writing parsers in LiveScript. It takes advantage of LiveScript's unique syntax to allow you to define your parser in code in a clear, declarative manner. Parsers are built up from simple rules which are composed like building blocks to create the much more complex final product. has no dependencies, but works best with LiveScript's syntax. requires the entire string be loaded into memory, so it's probably not the best choice for parsing large streams of data.


Just install via NPM:

npm install --save parse-ls

You can either require the entire library as variable: parse = require 'parse-ls', or just the rules you need: { char, $or, any, except, then-keep, then-ignore, many, join-string, convert-rule-to-function } = require 'parse-ls'. With the former, you'll need to prefix every rule with parse.. All examples and documentation use the latter, preferred method.

A Short Example

Here's a parser that would parse something like a Javascript string literal (with double quotes). For a heavily commented version of this code, see For an example that will do both single and double quotes, see

{ char, any, invert, then-keep, then-ignore, many, join-string, convert-rule-to-function } = require 'parse-ls'

quote = char '"'
slash = char '\\'
escapable = any quote, slash
escaped-character = slash |> then-keep escapable
unescaped-character = invert escapable
valid-character = any escaped-character, unescaped-character
inner-content = valid-character |> many |> join-string
string-rule = quote
	|> then-keep inner-content
	|> then-ignore quote

parse-function = convert-rule-to-function string-rule
result = parse-function '"A \\"quoted\\" string with \\\\ slahes"'

eq 'A "quoted" string with \\ slahes', result

Recent Changes

Changed from an object-based input to an interface based input. If you were creating input objects by hand, you'll need to change your process. If using to-input, no change should be necessary.

Updated any to accept a list of rules and succeed with the first one that succeeds (or fail if none succeed). Removed $or since it now duplicates functionality in any.

Added invert to invert the result of any rule. Removed do-until and except, as these can be easily done with the invert rule.

What is a Rule?

In, a rule is a simple function that accepts an input and returns a result, with either a success status or a failure status.

The input is any object with the following functions:

  • current: Returns the current token in the stream (in a string example, this would be a single character)
  • at-eof: Returns true if the input has reached the end.
  • next: Returns a new input object advanced one token
  • pos: Returns an object with the line number and column number of the current position (e.g. {line:15, column:3}). This is used when errors are incurred.

Rules should return output in one of the following formats. For a failure, the rule returns failure status, the successfully parsed input so far (in the same format as above), and any last known parsed value if applicable:

	"success": false,
	"message": "expected 'z'",
	"lastSuccess": lastSuccessfullyProcessedInput,
	"lastValue": "a"

For a success:

	"success": true,
	"value": "[the parsed value]",
	"remaining": theNextInputObjectToProcess

The success result has a true success status, contains the parsed value (which could be any thing: a string, an array, an object, etc.), and the remaining input after the rule has consumed input.

For example, he's a simple rule that will match any numeric digit passed to it, written in LiveScript:

matches-digit = (input) ->
	# if we're at the end of the input, return failure.
	return { success: false, message: 'unexpected end of file', last-success: input } if (!)

	# get the current char
	current-character = input.current!

	# see if it's in range
	is-in-range = current-character >= \0 && current-character <= \9

	# Return a failure result if it's not in range.
	if !is-in-range
		{ success: false, message: 'expected a digit', last-success: input }
		# Otherwise, return a success with the matched character as a
		# value.  Also return a new input with the index incremented by
		# 1, which is how many characters of input this rule consumed.
			success: true,
			value: current-character,

There's a lot of plumbing in this rule that's already taken care of in the framework, and this could much more succinctly be written as:

matches-digit = (simple -> it >= \0 and it <= \9) |> with-error-message 'expected a digit'

Take a look at the source file and the unit tests for more examples.

Available Rules

These are the rules that are included with To see basic examples of these rules in use, check out the unit tests or

to-input (string): Rules require the input be a specific format. This is a convenience method to convert strings (or even an array of tokens) to that format.

to-backwards-input (string): Like to-input, except the string or array of tokens is iterated backwards.

simple (delegate) - The most basic rule. It accepts a delegate that accepts one parameter (the individual character being tested), and returns true if the letter is matched, otherwise false.

with-error-message(message) - A rule to change the error message from the last rule (if it fails). This is useful for rules like simple that just return generic error messages. For example, an error message from match-a = simple (c) -> c == \a would simply say simple rule failed. To make the error message more clear, you could do this: `match-a = (simple (c) -> c == \a) |> with-error-message 'expected "a"``.

any (rules...) - If passed a set of rules as arguments, it will return the value from whichever rule matches first or fail. If no rules are passed, it will match any token and return it as the value.

char (character) - Matches an individual, specific character.

map (delegate) - Accepts a delegate to convert the current result to a different form.

debug (delegate) - Injects code into the parsing pipeline without effecting the output. Useful for putting in debugging console.log statements.

$then (rule, delegate) - Accepts a rule and delegate. If the prior rule in the chain succeeds, and the rule passed into this function succeeds, the results of both rules are passed as the two arguments to the delegate. The result of the delegate becomes the parsed value.

then-keep (rule) - $then shorthand that keeps the result of the second rule and throws away the first.

then-ignore (rule) - $then shorthand that keeps the result of the first rule and throws away the second.

then-concat (rule) - $then shorthand to add/concatenate the results of the previous and next rule in the chain.

then-null (rule) - $then shorthand to execute the next rule in the chain, but throw away all current results and return null as the value.

then-array-concat (rule) - $then shorthand to concatenate the results of the next rule under the assumption that both rules return arrays.

then-set (name, rule) - $then shorthand that sets a property on the current result to the value returned by the next rule. For example: headers |> as-object-with-value 'requestHeader' |> then-set 'requestDomain', domain might return { requestHeader: 'Some value', requestDomain: 'other value' }

many () - Attempts a rule repeatedly until it fails, returning results in an array. Rules that fail immediately still succeed with empty arrays.

times (integer) - Attempts a rule a set number of times. A rule can match more than the specified times, but will only be applied the specified number of times. If the input does not match enough times, the rule fails. May be useful for parsing fixed-width fields, like dates in a YYYY-MM-DD format. A rule requiring 0 matches will always succeed.

at-least (count) - Like many, but requires the rule match at least as many times as count - will continue to match even after count matches are met.

at-least-once () - Like many, but requires the rule match at least once. The equivalent of at-least 1.

join-string () - Convenience method for joining an array result into a string. The equivalent of map -> it.join ''.

as-array () - Convenience method for changing a single result into an array of 1 result. Useful when the following rules return arrays and you wish to concatenate them. For example, an identifier might allow only letters for the first character, followed by letters or numbers, might look like: (letter |> as-array) |> then-array-concat (letter-or-number |> many). Equivalent to map -> [it], but potentially more readable.

as-object-with-value (name) - Converts a result into an object with a property of name that is the result. For example, headers |> as-object-with-value 'requestHeader' might return { requestHeader: 'Some value' }

sequence (array-of-rules) - Accepts an array of rules and requires that all rules succeed. Returns the results of each rule in an array.

text (string) - Matches a string exactly and returns it as the result.

maybe () - Makes the preceding rule optional. If the rule doesn't match, a success result with a null value is returned.

invert (rule) - Inverts a rule, accepting anything that doesn't match the rule.

expect-end () - A rule that expects the input to be at the end. It succeeds if the input is at the end, otherwise fails.

end () - Convenience method to tack on to a rule chain to ensure input is completely consumed.

delay - Allows you to dynamically return a rule during the parse process, rather than during parser construction. Useful for circular-referencing rules. See below:

command  = null
command-delay = delay -> command
command-block = char '{'
	|> then-keep (command-delay |> many)
	|> then-ignore (char '}')
command = get-command |> $or set-command |> $or command-block

always (value) - Always returns a result without consuming input. Useful for optional values or starting off with a particular object as the value.

always-new (callback) - Always returns the result of the callback without consuming input. Useful for returning new objects with each execution of the rule.

parse () - Attempts to parse the input, returns the value if successful, otherwise throws an error.

convert-rule-to-function () - Converts a rule to a function which either returns a value or throws an error if the value could not be parsed.

Utility Functions

Included are a few utility functions that aren't exactly rules, but may make writing custom rules more clear.

input-at-eof (input) - Accepts an input object and returns true if the index is at or beyond the length of the input.

input-next (input) - Accepts an input object and returns a new input object with the index incremented by one.

input-current-letter (input) - Accepts an input object and returns the char at the current index.

pass (value, remaining) - Accepts a parsed value and the remaining input (after a rule has run) and returns a result object.

fail (message, last-successful-input, parsed-value) - Accepts an error message, an input object, and optionally a parsed value. The error message describes what failed, the input object represents the last successfully parsed input, and the parsed value (if provided) is the last successfully parsed value.

Also one function to make error reporting more clear:

line-and-column ({string, index}) - Accepts a string and an index (an input object), and calculates the line and column number of the index. Useful for returning an error position in a format that is more friendly for text editors.


  • More documentation with more complete examples that make use of all included rules.
  • Example of an object that implements the input interface

About is a library for writing parsers in LiveScript.




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