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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package snippet implements the specification for the LSP snippet format.
// Snippets are "tab stop" templates returned as an optional attribute of LSP
// completion candidates. As the user presses tab, they cycle through a series of
// tab stops defined in the snippet. Each tab stop can optionally have placeholder
// text, which can be pre-selected by editors. For a full description of syntax
// and features, see "Snippet Syntax" at
// A typical snippet looks like "foo(${1:i int}, ${2:s string})".
package snippet
import (
// A Builder is used to build an LSP snippet piecemeal.
// The zero value is ready to use. Do not copy a non-zero Builder.
type Builder struct {
// currentTabStop is the index of the previous tab stop. The
// next tab stop will be currentTabStop+1.
currentTabStop int
sb strings.Builder
// Escape characters defined in under "Grammar".
var replacer = strings.NewReplacer(
`\`, `\\`,
`}`, `\}`,
`$`, `\$`,
func (b *Builder) WriteText(s string) {
replacer.WriteString(&, s)
// WritePlaceholder writes a tab stop and placeholder value to the Builder.
// The callback style allows for creating nested placeholders. To write an
// empty tab stop, provide a nil callback.
func (b *Builder) WritePlaceholder(fn func(*Builder)) {
fmt.Fprintf(&, "${%d:", b.nextTabStop())
if fn != nil {
// WriteFinalTabstop marks where cursor ends up after the user has
// cycled through all the normal tab stops. It defaults to the
// character after the snippet.
func (b *Builder) WriteFinalTabstop() {
fmt.Fprint(&, "$0")
// In addition to '\', '}', and '$', snippet choices also use '|' and ',' as
// meta characters, so they must be escaped within the choices.
var choiceReplacer = strings.NewReplacer(
`\`, `\\`,
`}`, `\}`,
`$`, `\$`,
`|`, `\|`,
`,`, `\,`,
// WriteChoice writes a tab stop and list of text choices to the Builder.
// The user's editor will prompt the user to choose one of the choices.
func (b *Builder) WriteChoice(choices []string) {
fmt.Fprintf(&, "${%d|", b.nextTabStop())
for i, c := range choices {
if i != 0 {',')
choiceReplacer.WriteString(&, c)
// String returns the built snippet string.
func (b *Builder) String() string {
// nextTabStop returns the next tab stop index for a new placeholder.
func (b *Builder) nextTabStop() int {
// Tab stops start from 1, so increment before returning.
return b.currentTabStop