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// Copyright 2017 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 catmsg contains support types for package x/text/message/catalog.
// This package contains the low-level implementations of Message used by the
// catalog package and provides primitives for other packages to implement their
// own. For instance, the plural package provides functionality for selecting
// translation strings based on the plural category of substitution arguments.
// # Encoding and Decoding
// Catalogs store Messages encoded as a single string. Compiling a message into
// a string both results in compacter representation and speeds up evaluation.
// A Message must implement a Compile method to convert its arbitrary
// representation to a string. The Compile method takes an Encoder which
// facilitates serializing the message. Encoders also provide more context of
// the messages's creation (such as for which language the message is intended),
// which may not be known at the time of the creation of the message.
// Each message type must also have an accompanying decoder registered to decode
// the message. This decoder takes a Decoder argument which provides the
// counterparts for the decoding.
// # Renderers
// A Decoder must be initialized with a Renderer implementation. These
// implementations must be provided by packages that use Catalogs, typically
// formatting packages such as x/text/message. A typical user will not need to
// worry about this type; it is only relevant to packages that do string
// formatting and want to use the catalog package to handle localized strings.
// A package that uses catalogs for selecting strings receives selection results
// as sequence of substrings passed to the Renderer. The following snippet shows
// how to express the above example using the message package.
// message.Set(language.English, "You are %d minute(s) late.",
// catalog.Var("minutes", plural.Select(1, "one", "minute")),
// catalog.String("You are %[1]d ${minutes} late."))
// p := message.NewPrinter(language.English)
// p.Printf("You are %d minute(s) late.", 5) // always 5 minutes late.
// To evaluate the Printf, package message wraps the arguments in a Renderer
// that is passed to the catalog for message decoding. The call sequence that
// results from evaluating the above message, assuming the person is rather
// tardy, is:
// Render("You are %[1]d ")
// Arg(1)
// Render("minutes")
// Render(" late.")
// The calls to Arg is caused by the plural.Select execution, which evaluates
// the argument to determine whether the singular or plural message form should
// be selected. The calls to Render reports the partial results to the message
// package for further evaluation.
package catmsg
import (
// A Handle refers to a registered message type.
type Handle int
// A Handler decodes and evaluates data compiled by a Message and sends the
// result to the Decoder. The output may depend on the value of the substitution
// arguments, accessible by the Decoder's Arg method. The Handler returns false
// if there is no translation for the given substitution arguments.
type Handler func(d *Decoder) bool
// Register records the existence of a message type and returns a Handle that
// can be used in the Encoder's EncodeMessageType method to create such
// messages. The prefix of the name should be the package path followed by
// an optional disambiguating string.
// Register will panic if a handle for the same name was already registered.
func Register(name string, handler Handler) Handle {
defer mutex.Unlock()
if _, ok := names[name]; ok {
panic(fmt.Errorf("catmsg: handler for %q already exists", name))
h := Handle(len(handlers))
names[name] = h
handlers = append(handlers, handler)
return h
// These handlers require fixed positions in the handlers slice.
const (
msgVars Handle = iota
// Leave some arbitrary room for future expansion: 20 should suffice.
numInternal = 20
const prefix = ""
var (
// TODO: find a more stable way to link handles to message types.
mutex sync.Mutex
names = map[string]Handle{
prefix + "Vars": msgVars,
prefix + "First": msgFirst,
prefix + "Raw": msgRaw,
prefix + "String": msgString,
prefix + "Affix": msgAffix,
handlers = make([]Handler, numInternal)
func init() {
// This handler is a message type wrapper that initializes a decoder
// with a variable block. This message type, if present, is always at the
// start of an encoded message.
handlers[msgVars] = func(d *Decoder) bool {
blockSize := int(d.DecodeUint())
d.vars =[:blockSize] =[blockSize:]
return d.executeMessage()
// First takes the first message in a sequence that results in a match for
// the given substitution arguments.
handlers[msgFirst] = func(d *Decoder) bool {
for !d.Done() {
if d.ExecuteMessage() {
return true
return false
handlers[msgRaw] = func(d *Decoder) bool {
return true
// A String message alternates between a string constant and a variable
// substitution.
handlers[msgString] = func(d *Decoder) bool {
for !d.Done() {
if str := d.DecodeString(); str != "" {
if d.Done() {
return true
handlers[msgAffix] = func(d *Decoder) bool {
// TODO: use an alternative method for common cases.
prefix := d.DecodeString()
suffix := d.DecodeString()
if prefix != "" {
ret := d.ExecuteMessage()
if suffix != "" {
return ret
var (
// ErrIncomplete indicates a compiled message does not define translations
// for all possible argument values. If this message is returned, evaluating
// a message may result in the ErrNoMatch error.
ErrIncomplete = errors.New("catmsg: incomplete message; may not give result for all inputs")
// ErrNoMatch indicates no translation message matched the given input
// parameters when evaluating a message.
ErrNoMatch = errors.New("catmsg: no translation for inputs")
// A Message holds a collection of translations for the same phrase that may
// vary based on the values of substitution arguments.
type Message interface {
// Compile encodes the format string(s) of the message as a string for later
// evaluation.
// The first call Compile makes on the encoder must be EncodeMessageType.
// The handle passed to this call may either be a handle returned by
// Register to encode a single custom message, or HandleFirst followed by
// a sequence of calls to EncodeMessage.
// Compile must return ErrIncomplete if it is possible for evaluation to
// not match any translation for a given set of formatting parameters.
// For example, selecting a translation based on plural form may not yield
// a match if the form "Other" is not one of the selectors.
// Compile may return any other application-specific error. For backwards
// compatibility with package like fmt, which often do not do sanity
// checking of format strings ahead of time, Compile should still make an
// effort to have some sensible fallback in case of an error.
Compile(e *Encoder) error
// Compile converts a Message to a data string that can be stored in a Catalog.
// The resulting string can subsequently be decoded by passing to the Execute
// method of a Decoder.
func Compile(tag language.Tag, macros Dictionary, m Message) (data string, err error) {
// TODO: pass macros so they can be used for validation.
v := &Encoder{inBody: true} // encoder for variables
v.root = v
e := &Encoder{root: v, parent: v, tag: tag} // encoder for messages
err = m.Compile(e)
// This package serves te message package, which in turn is meant to be a
// drop-in replacement for fmt. With the fmt package, format strings are
// evaluated lazily and errors are handled by substituting strings in the
// result, rather then returning an error. Dealing with multiple languages
// makes it more important to check errors ahead of time. We chose to be
// consistent and compatible and allow graceful degradation in case of
// errors.
buf := e.buf[stripPrefix(e.buf):]
if len(v.buf) > 0 {
// Prepend variable block.
b := make([]byte, 1+maxVarintBytes+len(v.buf)+len(buf))
b[0] = byte(msgVars)
b = b[:1+encodeUint(b[1:], uint64(len(v.buf)))]
b = append(b, v.buf...)
b = append(b, buf...)
buf = b
if err == nil {
err = v.err
return string(buf), err
// FirstOf is a message type that prints the first message in the sequence that
// resolves to a match for the given substitution arguments.
type FirstOf []Message
// Compile implements Message.
func (s FirstOf) Compile(e *Encoder) error {
err := ErrIncomplete
for i, m := range s {
if err == nil {
return fmt.Errorf("catalog: message argument %d is complete and blocks subsequent messages", i-1)
err = e.EncodeMessage(m)
return err
// Var defines a message that can be substituted for a placeholder of the same
// name. If an expression does not result in a string after evaluation, Name is
// used as the substitution. For example:
// Var{
// Name: "minutes",
// Message: plural.Select(1, "one", "minute"),
// }
// will resolve to minute for singular and minutes for plural forms.
type Var struct {
Name string
Message Message
var errIsVar = errors.New("catmsg: variable used as message")
// Compile implements Message.
// Note that this method merely registers a variable; it does not create an
// encoded message.
func (v *Var) Compile(e *Encoder) error {
if err := e.addVar(v.Name, v.Message); err != nil {
return err
// Using a Var by itself is an error. If it is in a sequence followed by
// other messages referring to it, this error will be ignored.
return errIsVar
// Raw is a message consisting of a single format string that is passed as is
// to the Renderer.
// Note that a Renderer may still do its own variable substitution.
type Raw string
// Compile implements Message.
func (r Raw) Compile(e *Encoder) (err error) {
// Special case: raw strings don't have a size encoding and so don't use
// EncodeString.
e.buf = append(e.buf, r...)
return nil
// String is a message consisting of a single format string which contains
// placeholders that may be substituted with variables.
// Variable substitutions are marked with placeholders and a variable name of
// the form ${name}. Any other substitutions such as Go templates or
// printf-style substitutions are left to be done by the Renderer.
// When evaluation a string interpolation, a Renderer will receive separate
// calls for each placeholder and interstitial string. For example, for the
// message: "%[1]v ${invites} %[2]v to ${their} party." The sequence of calls
// is:
// d.Render("%[1]v ")
// d.Arg(1)
// d.Render(resultOfInvites)
// d.Render(" %[2]v to ")
// d.Arg(2)
// d.Render(resultOfTheir)
// d.Render(" party.")
// where the messages for "invites" and "their" both use a plural.Select
// referring to the first argument.
// Strings may also invoke macros. Macros are essentially variables that can be
// reused. Macros may, for instance, be used to make selections between
// different conjugations of a verb. See the catalog package description for an
// overview of macros.
type String string
// Compile implements Message. It parses the placeholder formats and returns
// any error.
func (s String) Compile(e *Encoder) (err error) {
msg := string(s)
const subStart = "${"
hasHeader := false
p := 0
b := []byte{}
for {
i := strings.Index(msg[p:], subStart)
if i == -1 {
b = append(b, msg[p:p+i]...)
p += i + len(subStart)
if i = strings.IndexByte(msg[p:], '}'); i == -1 {
b = append(b, "$!(MISSINGBRACE)"...)
err = fmt.Errorf("catmsg: missing '}'")
p = len(msg)
name := strings.TrimSpace(msg[p : p+i])
if q := strings.IndexByte(name, '('); q == -1 {
if !hasHeader {
hasHeader = true
b = b[:0]
} else if j := strings.IndexByte(name[q:], ')'); j == -1 {
// TODO: what should the error be?
b = append(b, "$!(MISSINGPAREN)"...)
err = fmt.Errorf("catmsg: missing ')'")
} else if x, sErr := strconv.ParseUint(strings.TrimSpace(name[q+1:q+j]), 10, 32); sErr != nil {
// TODO: handle more than one argument
b = append(b, "$!(BADNUM)"...)
err = fmt.Errorf("catmsg: invalid number %q", strings.TrimSpace(name[q+1:q+j]))
} else {
if !hasHeader {
hasHeader = true
e.EncodeSubstitution(name[:q], int(x))
b = b[:0]
p += i + 1
b = append(b, msg[p:]...)
if !hasHeader {
// Simplify string to a raw string.
} else if len(b) > 0 {
return err
// Affix is a message that adds a prefix and suffix to another message.
// This is mostly used add back whitespace to a translation that was stripped
// before sending it out.
type Affix struct {
Message Message
Prefix string
Suffix string
// Compile implements Message.
func (a Affix) Compile(e *Encoder) (err error) {
// TODO: consider adding a special message type that just adds a single
// return. This is probably common enough to handle the majority of cases.
// Get some stats first, though.
return nil