JavaScript

To get an overview of how to use Canopy with JavaScript, consider this example of a simplified grammar for URLs:

url.peg
grammar URL
  url       <-  scheme "://" host pathname search hash?
  scheme    <-  "http" "s"?
  host      <-  hostname port?
  hostname  <-  segment ("." segment)*
  segment   <-  [a-z0-9-]+
  port      <-  ":" [0-9]+
  pathname  <-  "/" [^ ?]*
  search    <-  ("?" query:[^ #]*)?
  hash      <-  "#" [^ ]*

We can compile this grammar into a JavaScript module using canopy:

$ canopy url.peg --lang js

This creates a file called url.js that contains all the parser logic, and it works in Node and in the browser. The --output option can be used to override the default location:

$ canopy url.peg --lang js --output some/dir/url

This will write the generated parser into the file some/dir/url.js.

Let’s try our parser out:

const url = require('./url')

let tree = url.parse('http://example.com/search?q=hello#page=1')

for (let node of tree) {
  console.log(node.offset, node.text)
}

/*  prints:

    0 'http'
    4 '://'
    7 'example.com'
    18 '/search'
    25 '?q=hello'
    33 '#page=1'      */

This little example shows a few important things:

You invoke the parser by calling the module’s parse() function with a string. In the browser, you can call URL.parse() rather than using require(); Canopy creates a global named after the grammar.

The parse() method returns a tree of nodes.

Each node has three properties:

Walking the parse tree

You can use elements to walk into the structure of the tree:

console.log(tree.elements[4].elements[1].text)
// -> 'q=hello'

Or, you can use the labels that Canopy generates, which can make your code clearer:

console.log(tree.search.query.text)
// -> 'q=hello'

Parsing errors

If you give the parser an input text that does not match the grammar, a SyntaxError is thrown. The error message will list any of the strings or character classes the parser was expecting to find at the furthest position it got to, along with the rule those expectations come from, and it will highlight the line of the input where the syntax error occurs.

url.parse('https://example.com./')

// SyntaxError: Line 1: expected one of:
//
//     - [a-z0-9-] from URL::segment
//
//      1 | https://example.com./
//                              ^

Implementing actions

Say you have a grammar that uses action annotations, for example:

maps.peg
grammar Maps
  map     <-  "{" string ":" value "}" %make_map
  string  <-  "'" [^']* "'" %make_string
  value   <-  list / number
  list    <-  "[" value ("," value)* "]" %make_list
  number  <-  [0-9]+ %make_number

In JavaScript, you give the action functions to the parser by using the actions option, which should be an object implementing the named actions:

const maps = require('./maps')

const actions = {
  make_map (input, start, end, [_, key, __, value]) {
    let map = {}
    map[key] = value
    return map
  },

  make_string (input, start, end, [_, string]) {
    return string.text
  },

  make_list (input, start, end, [_, first, rest]) {
    rest = [...rest].map((el) => el.value)
    return [first, ...rest]
  },

  make_number (input, start, end, _) {
    return parseInt(input.substring(start, end), 10)
  }
}

let result = maps.parse("{'ints':[1,2,3]}", { actions })

console.log(result)
// -> { ints: [ 1, 2, 3 ] }

Extended node types

Say you have a grammar that contains type annotations:

words.peg
grammar Words
  root  <-  first:"foo" second:"bar" <Extension>

To use this parser, you must pass in an object containing implementations of the named types via the types option. Each defined type contains the methods that will be added to the nodes.

const words = require('./words')

const types = {
  Extension: {
    convert () {
      return this.first.text + this.second.text.toUpperCase()
    }
  }
}

words.parse('foobar', { types }).convert()
// -> 'fooBAR'