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// Copyright (C) 2014 The Android Open Source Project
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
Package parse provides support functionality for writing scannerless parsers.
The main entry point is the Parse function.
It works by setting up a parser on the supplied content, and then invoking the
supplied root parsing function. The CST is built for you inside the ParseLeaf
and ParseBranch methods of the parser, but it is up to the supplied parsing
functions to hold on to the CST if you want it, and also to build the AST.
package parse
// BranchParser is a function that is passed to ParseBranch. It is handed the
// Branch to fill in and the Parser to fill it from, and must either succeed or
// add an error to the parser.
type BranchParser func(p *Parser, cst *Branch)
// LeafParser is a function that is passed to ParseLeaf. It is handed the
// Leaf to fill in and the Parser to fill it from, and must either succeed or
// add an error to the parser.
type LeafParser func(p *Parser, cst *Leaf)
// Parse is the main entry point to the parse library.
// Given a root parse function, the input string and the Skip controller, it builds a
// and initializes a Parser, runs the root using it, verifies it worked
// correctly and then returns the errors generated if any.
func Parse(root BranchParser, data string, skip Skip) []Error {
p := &Parser{
skip: skip,
return p.Errors
// Parser contains all the context needed while parsing.
// They are built for you by the Parse function.
type Parser struct {
Reader // The token reader for this parser.
Errors ErrorList // The set of errors generated during the parse.
skip Skip // The whitespace skipping function.
prefix Separator // The currently skipped prefix separator.
suffix Separator // The currently skipped suffix separator.
last Node // The last node fully parsed, potential suffix target
func (p *Parser) parse(root BranchParser) {
defer func() {
err := recover()
if err != nil && err != AbortParse {
anchor := &Branch{}
anchor.AddPrefix(p.skip(p, SkipPrefix))
root(p, anchor)
if len(p.suffix) > 0 {
if len(p.prefix) > 0 {
// This is a trailing "prefix" for a node that is never going to arrive, so
// we treat it as a suffix of the cst root instead.
if !p.IsEOF() {
p.Error("Unexpected input at end of parse")
func (p *Parser) addChild(in *Branch, child Node) {
if p.suffix != nil {
p.suffix = nil
p.prefix = nil
in.Children = append(in.Children, child)
// ParseLeaf adds a new Leaf to cst and then calls the do function to
// parse the Leaf.
// If do is nil, a leaf will be built with the current unconsumed input.
func (p *Parser) ParseLeaf(cst *Branch, do LeafParser) {
l := &Leaf{}
p.addChild(cst, l)
if do != nil {
do(p, l)
if p.offset != p.cursor {
p.suffix = p.skip(p, SkipSuffix)
p.prefix = p.skip(p, SkipPrefix)
p.last = l
// ParseLeaf adds a new Branch to cst and then calls the do function to
// parse the branch.
// This is called recursively to build the node tree.
func (p *Parser) ParseBranch(cst *Branch, do BranchParser) {
if p.offset != p.cursor {
p.Error("Starting ParseBranch with parsed but unconsumed tokens")
b := &Branch{}
p.addChild(cst, b)
do(p, b)
if p.offset != p.cursor {
p.Error("Finishing ParseBranch with parsed but unconsumed tokens")
p.last = b
// Error adds a new error to the parser error list. It will attempt to consume
// a token from the reader to act as a place holder, and also to ensure
// progress is made in the presence of errors.
func (p *Parser) Error(message string, args ...interface{}) {
p.Errors.Add(&p.Reader, nil, message, args...)
// ErrorAt is like Error, except because it is handed a fragment, it will not
// try to consume anything itself.
func (p *Parser) ErrorAt(at Fragment, message string, args ...interface{}) {
p.Errors.Add(&p.Reader, at, message, args...)
// Expected is a wrapper around p.ErrorAt for the very common case of an unexpected
// input. It uses value as the expected input, and parses a token of the stream
// for the unexpected actual input.
func (p *Parser) Expected(value string) {
invalid := &fragment{}
p.Errors.Add(&p.Reader, invalid, "Expected \"%s\" got \"%s\"", value, invalid.Token().String())