Allow custom text for printed strings

[m-horky]

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What problem does this solve?

I'd like to be able to change any (English) string that is printed to the console. It would be useful for translating applications (is this issue a duplicate of #980?) -- not being able to alter texts during runtime prevents that.

Solution description

Similarly to how it is possible to change texts of VersionFlag or HelpFlag, I'd like to be able to access:

• command.go: Incorrect Usage: in Run()
• context.go: no such flag -%s in Set()
• errors.go: most of the error messages
• flag.go: default in formatDefault()
• parse.go: const lag provided but not defined: -
• and possibly more

Describe alternatives you've considered

An alternative could be to add full i18n support as per #980. But that also comes with its set of challenges -- mainly how to allow external applications to extend or include the translations of this project in theirs (languages not supported by this package, or to change the provided translation for whatever reasons).

[dearchap]

@m-horky Thanks for this. yes i18n support is planned. If you have any ideas on how to do this it would be greatly appreciated

[m-horky]

Most of the library is already translatable, which should work well until some library breaking changes.

Assuming translation function T() (I'm using gotext's gotext.Get() aliased to T) that takes in English string and returns appropriately translated string, you can translate default flags, suggestions and templates:

VersionFlag, HelpFlag, SuggestDidYouMean can be simply overwritten.

cli.VersionFlag = &cli.BoolFlag{
	Name:    "version",
	Aliases: []string{"v"},
	Usage:   T("show version and exit"),
}
cli.SuggestDidYouMeanTemplate = T("Did you mean %q?")

AppHelp can be redefined

// ui/help.go
func GetAppHelp() string {
	templateHeader := "{{.Name}}\n  {{.Usage}}"

	templateUsage := T("USAGE") + "\n  " + "{{.HelpName}} " + T("[global options]") + " " + T("COMMAND [command  options]") + " " + fmt.Sprintf("{{if .ArgsUsage}}{{.ArgsUsage}}{{else}}%s{{end}}", T("[arguments]"))
	templateCommands := T("COMMANDS") + `{{range .Commands}}{{if not .HideHelp}}  {{ "\n  " }}{{join .Names ", "}}{{ "\t"}}{{.Usage}}{{end}}{{end}}`
	templateGlobals := T("GLOBAL OPTIONS") + `{{range .VisibleFlags}}{{ "\n  " }}{{.}}{{end}}`
	templateVersion := fmt.Sprintf("%s\n  {{.Version}}", T("VERSION"))

	return templateHeader + "\n\n" + templateUsage + "\n\n" + templateCommands + "\n\n" + templateGlobals + "\n\n" + templateVersion + "\n"
}
// main.go
cli.AppHelpTemplate = ui.GetAppHelp()

...which is not pretty, but gets its job done.

---

As noted in the opening issue comment, some strings cannot be translated like that, because they are hardcoded in the functions or are private.

The solution that seems the easiest to me is to move all strings that are printed or returned into public variables that can be overwritten. Just like SuggestDidYouMeanTemplate, each could be a variable anyone can overwrite if they need to.

I'm not sure how to deal with the longer templates -- while it is possible to overwrite whole string, it would be cool it it was possible to split them into individual blocks (header, usage, commands) that could be translated individually, and developers using the library wouldn't need to deal with the conditionals inside of the template. But that's rather a nice to have, just having a way to translate any string would be a huge step forward into i18n-supportable space.

[dearchap]

@m-horky yes it looks like a duplicate of #980 . I will get working on this. Thanks for raising the issue