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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 catalog defines collections of translated format strings.
// This package mostly defines types for populating catalogs with messages. The
// catmsg package contains further definitions for creating custom message and
// dictionary types as well as packages that use Catalogs.
// Package catalog defines various interfaces: Dictionary, Loader, and Message.
// A Dictionary maintains a set of translations of format strings for a single
// language. The Loader interface defines a source of dictionaries. A
// translation of a format string is represented by a Message.
// Catalogs
// A Catalog defines a programmatic interface for setting message translations.
// It maintains a set of per-language dictionaries with translations for a set
// of keys. For message translation to function properly, a translation should
// be defined for each key for each supported language. A dictionary may be
// underspecified, though, if there is a parent language that already defines
// the key. For example, a Dictionary for "en-GB" could leave out entries that
// are identical to those in a dictionary for "en".
// Messages
// A Message is a format string which varies on the value of substitution
// variables. For instance, to indicate the number of results one could want "no
// results" if there are none, "1 result" if there is 1, and "%d results" for
// any other number. Catalog is agnostic to the kind of format strings that are
// used: for instance, messages can follow either the printf-style substitution
// from package fmt or use templates.
// A Message does not substitute arguments in the format string. This job is
// reserved for packages that render strings, such as message, that use Catalogs
// to selected string. This separation of concerns allows Catalog to be used to
// store any kind of formatting strings.
// Selecting messages based on linguistic features of substitution arguments
// Messages may vary based on any linguistic features of the argument values.
// The most common one is plural form, but others exist.
// Selection messages are provided in packages that provide support for a
// specific linguistic feature. The following snippet uses plural.Select:
// catalog.Set(language.English, "You are %d minute(s) late.",
// plural.Select(1,
// "one", "You are 1 minute late.",
// "other", "You are %d minutes late."))
// In this example, a message is stored in the Catalog where one of two messages
// is selected based on the first argument, a number. The first message is
// selected if the argument is singular (identified by the selector "one") and
// the second message is selected in all other cases. The selectors are defined
// by the plural rules defined in CLDR. The selector "other" is special and will
// always match. Each language always defines one of the linguistic categories
// to be "other." For English, singular is "one" and plural is "other".
// Selects can be nested. This allows selecting sentences based on features of
// multiple arguments or multiple linguistic properties of a single argument.
// String interpolation
// There is often a lot of commonality between the possible variants of a
// message. For instance, in the example above the word "minute" varies based on
// the plural catogory of the argument, but the rest of the sentence is
// identical. Using interpolation the above message can be rewritten as:
// catalog.Set(language.English, "You are %d minute(s) late.",
// catalog.Var("minutes",
// plural.Select(1, "one", "minute", "other", "minutes")),
// catalog.String("You are %[1]d ${minutes} late."))
// Var is defined to return the variable name if the message does not yield a
// match. This allows us to further simplify this snippet to
// catalog.Set(language.English, "You are %d minute(s) late.",
// catalog.Var("minutes", plural.Select(1, "one", "minute")),
// catalog.String("You are %d ${minutes} late."))
// Overall this is still only a minor improvement, but things can get a lot more
// unwieldy if more than one linguistic feature is used to determine a message
// variant. Consider the following example:
// // argument 1: list of hosts, argument 2: list of guests
// catalog.Set(language.English, "%[1]v invite(s) %[2]v to their party.",
// catalog.Var("their",
// plural.Select(1,
// "one", gender.Select(1, "female", "her", "other", "his"))),
// catalog.Var("invites", plural.Select(1, "one", "invite"))
// catalog.String("%[1]v ${invites} %[2]v to ${their} party.")),
// Without variable substitution, this would have to be written as
// // argument 1: list of hosts, argument 2: list of guests
// catalog.Set(language.English, "%[1]v invite(s) %[2]v to their party.",
// plural.Select(1,
// "one", gender.Select(1,
// "female", "%[1]v invites %[2]v to her party."
// "other", "%[1]v invites %[2]v to his party."),
// "other", "%[1]v invites %[2]v to their party.")
// Not necessarily shorter, but using variables there is less duplication and
// the messages are more maintenance friendly. Moreover, languages may have up
// to six plural forms. This makes the use of variables more welcome.
// Different messages using the same inflections can reuse variables by moving
// them to macros. Using macros we can rewrite the message as:
// // argument 1: list of hosts, argument 2: list of guests
// catalog.SetString(language.English, "%[1]v invite(s) %[2]v to their party.",
// "%[1]v ${invites(1)} %[2]v to ${their(1)} party.")
// Where the following macros were defined separately.
// catalog.SetMacro(language.English, "invites", plural.Select(1, "one", "invite"))
// catalog.SetMacro(language.English, "their", plural.Select(1,
// "one", gender.Select(1, "female", "her", "other", "his"))),
// Placeholders use parentheses and the arguments to invoke a macro.
// Looking up messages
// Message lookup using Catalogs is typically only done by specialized packages
// and is not something the user should be concerned with. For instance, to
// express the tardiness of a user using the related message we defined earlier,
// the user may use the package message like so:
// p := message.NewPrinter(language.English)
// p.Printf("You are %d minute(s) late.", 5)
// Which would print:
// You are 5 minutes late.
// This package is UNDER CONSTRUCTION and its API may change.
package catalog // import ""
// TODO:
// Some way to freeze a catalog.
// - Locking on each lockup turns out to be about 50% of the total running time
// for some of the benchmarks in the message package.
// Consider these:
// - Sequence type to support sequences in user-defined messages.
// - Garbage collection: Remove dictionaries that can no longer be reached
// as other dictionaries have been added that cover all possible keys.
import (
// A Catalog holds translations for messages for supported languages.
type Catalog struct {
index store
macros store
type options struct{}
// An Option configures Catalog behavior.
type Option func(*options)
// TODO:
// // Catalogs specifies one or more sources for a Catalog.
// // Lookups are in order.
// // This can be changed inserting a Catalog used for setting, which implements
// // Loader, used for setting in the chain.
// func Catalogs(d ...Loader) Option {
// return nil
// }
// func Delims(start, end string) Option {}
// func Dict(tag language.Tag, d ...Dictionary) Option
// New returns a new Catalog.
func New(opts ...Option) *Catalog {
c := &Catalog{}
for _, o := range opts {
return c
// Languages returns all languages for which the Catalog contains variants.
func (c *Catalog) Languages() []language.Tag {
return c.index.languages()
// SetString is shorthand for Set(tag, key, String(msg)).
func (c *Catalog) SetString(tag language.Tag, key string, msg string) error {
return c.set(tag, key, &c.index, String(msg))
// Set sets the translation for the given language and key.
// When evaluation this message, the first Message in the sequence to msgs to
// evaluate to a string will be the message returned.
func (c *Catalog) Set(tag language.Tag, key string, msg ...Message) error {
return c.set(tag, key, &c.index, msg...)
// SetMacro defines a Message that may be substituted in another message.
// The arguments to a macro Message are passed as arguments in the
// placeholder the form "${foo(arg1, arg2)}".
func (c *Catalog) SetMacro(tag language.Tag, name string, msg ...Message) error {
return c.set(tag, name, &c.macros, msg...)
// ErrNotFound indicates there was no message for the given key.
var ErrNotFound = errors.New("catalog: message not found")
// A Message holds a collection of translations for the same phrase that may
// vary based on the values of substitution arguments.
type Message interface {
// String specifies a plain message string. It can be used as fallback if no
// other strings match or as a simple standalone message.
// It is an error to pass more than one String in a message sequence.
func String(name string) Message {
return catmsg.String(name)
// Var sets a variable that may be substituted in formatting patterns using
// named substitution of the form "${name}". The name argument is used as a
// fallback if the statements do not produce a match. The statement sequence may
// not contain any Var calls.
// The name passed to a Var must be unique within message sequence.
func Var(name string, msg ...Message) Message {
return &catmsg.Var{Name: name, Message: firstInSequence(msg)}
// firstInSequence is a message type that prints the first message in the
// sequence that resolves to a match for the given substitution arguments.
type firstInSequence []Message
func (s firstInSequence) Compile(e *catmsg.Encoder) error {
err := catmsg.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
// Context returns a Context for formatting messages.
// Only one Message may be formatted per context at any given time.
func (c *Catalog) Context(tag language.Tag, r catmsg.Renderer) *Context {
return &Context{
cat: c,
tag: tag,
dec: catmsg.NewDecoder(tag, r, &dict{&c.macros, tag}),
// A Context is used for evaluating Messages.
// Only one Message may be formatted per context at any given time.
type Context struct {
cat *Catalog
tag language.Tag
dec *catmsg.Decoder
// Execute looks up and executes the message with the given key.
// It returns ErrNotFound if no message could be found in the index.
func (c *Context) Execute(key string) error {
data, ok :=, key)
if !ok {
return ErrNotFound
return c.dec.Execute(data)