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Bumps [github.com/alecthomas/kong](https://github.com/alecthomas/kong) from 0.9.0 to 1.6.0. - [Commits](https://github.com/alecthomas/kong/compare/v0.9.0...v1.6.0) --- updated-dependencies: - dependency-name: github.com/alecthomas/kong dependency-type: direct:production update-type: version-update:semver-major ... Signed-off-by: dependabot[bot] <support@github.com>
766 lines
35 KiB
Markdown
766 lines
35 KiB
Markdown
<!-- markdownlint-disable MD013 MD033 -->
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<p align="center"><img width="90%" src="kong.png" /></p>
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# Kong is a command-line parser for Go
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[](http://godoc.org/github.com/alecthomas/kong) [](https://circleci.com/gh/alecthomas/kong) [](https://goreportcard.com/report/github.com/alecthomas/kong) [](https://gophers.slack.com/messages/CN9DS8YF3)
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- [Kong is a command-line parser for Go](#kong-is-a-command-line-parser-for-go)
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- [Version 1.0.0 Release](#version-100-release)
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- [Introduction](#introduction)
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- [Help](#help)
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- [Help as a user of a Kong application](#help-as-a-user-of-a-kong-application)
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- [Defining help in Kong](#defining-help-in-kong)
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- [Command handling](#command-handling)
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- [Switch on the command string](#switch-on-the-command-string)
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- [Attach a `Run(...) error` method to each command](#attach-a-run-error-method-to-each-command)
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- [Hooks: BeforeReset(), BeforeResolve(), BeforeApply(), AfterApply() and the Bind() option](#hooks-beforereset-beforeresolve-beforeapply-afterapply-and-the-bind-option)
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- [Flags](#flags)
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- [Commands and sub-commands](#commands-and-sub-commands)
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- [Branching positional arguments](#branching-positional-arguments)
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- [Positional arguments](#positional-arguments)
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- [Slices](#slices)
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- [Maps](#maps)
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- [Pointers](#pointers)
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- [Nested data structure](#nested-data-structure)
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- [Custom named decoders](#custom-named-decoders)
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- [Supported field types](#supported-field-types)
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- [Custom decoders (mappers)](#custom-decoders-mappers)
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- [Supported tags](#supported-tags)
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- [Plugins](#plugins)
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- [Dynamic Commands](#dynamic-commands)
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- [Variable interpolation](#variable-interpolation)
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- [Validation](#validation)
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- [Modifying Kong's behaviour](#modifying-kongs-behaviour)
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- [`Name(help)` and `Description(help)` - set the application name description](#namehelp-and-descriptionhelp---set-the-application-name-description)
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- [`Configuration(loader, paths...)` - load defaults from configuration files](#configurationloader-paths---load-defaults-from-configuration-files)
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- [`Resolver(...)` - support for default values from external sources](#resolver---support-for-default-values-from-external-sources)
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- [`*Mapper(...)` - customising how the command-line is mapped to Go values](#mapper---customising-how-the-command-line-is-mapped-to-go-values)
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- [`ConfigureHelp(HelpOptions)` and `Help(HelpFunc)` - customising help](#configurehelphelpoptions-and-helphelpfunc---customising-help)
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- [`Bind(...)` - bind values for callback hooks and Run() methods](#bind---bind-values-for-callback-hooks-and-run-methods)
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- [Other options](#other-options)
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## Version 1.0.0 Release
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Kong has been stable for a long time, so it seemed appropriate to cut a 1.0 release.
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There is one breaking change, [#436](https://github.com/alecthomas/kong/pull/436), which should effect relatively few users.
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## Introduction
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Kong aims to support arbitrarily complex command-line structures with as little developer effort as possible.
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To achieve that, command-lines are expressed as Go types, with the structure and tags directing how the command line is mapped onto the struct.
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For example, the following command-line:
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shell rm [-f] [-r] <paths> ...
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shell ls [<paths> ...]
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Can be represented by the following command-line structure:
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```go
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package main
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import "github.com/alecthomas/kong"
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var CLI struct {
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Rm struct {
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Force bool `help:"Force removal."`
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Recursive bool `help:"Recursively remove files."`
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Paths []string `arg:"" name:"path" help:"Paths to remove." type:"path"`
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} `cmd:"" help:"Remove files."`
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Ls struct {
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Paths []string `arg:"" optional:"" name:"path" help:"Paths to list." type:"path"`
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} `cmd:"" help:"List paths."`
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}
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func main() {
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ctx := kong.Parse(&CLI)
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switch ctx.Command() {
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case "rm <path>":
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case "ls":
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default:
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panic(ctx.Command())
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}
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}
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```
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## Help
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### Help as a user of a Kong application
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Every Kong application includes a `--help` flag that will display auto-generated help.
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eg.
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$ shell --help
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usage: shell <command>
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A shell-like example app.
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Flags:
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--help Show context-sensitive help.
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--debug Debug mode.
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Commands:
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rm <path> ...
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Remove files.
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ls [<path> ...]
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List paths.
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If a command is provided, the help will show full detail on the command including all available flags.
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eg.
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$ shell --help rm
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usage: shell rm <paths> ...
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Remove files.
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Arguments:
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<paths> ... Paths to remove.
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Flags:
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--debug Debug mode.
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-f, --force Force removal.
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-r, --recursive Recursively remove files.
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### Defining help in Kong
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Help is automatically generated from the command-line structure itself,
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including `help:""` and other tags. [Variables](#variable-interpolation) will
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also be interpolated into the help string.
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Finally, any command, or argument type implementing the interface
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`Help() string` will have this function called to retrieve more detail to
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augment the help tag. This allows for much more descriptive text than can
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fit in Go tags. [See \_examples/shell/help](./_examples/shell/help)
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#### Showing the _command_'s detailed help
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A command's additional help text is _not_ shown from top-level help, but can be displayed within contextual help:
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**Top level help**
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```bash
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$ go run ./_examples/shell/help --help
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Usage: help <command>
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An app demonstrating HelpProviders
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Flags:
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-h, --help Show context-sensitive help.
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--flag Regular flag help
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Commands:
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echo Regular command help
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```
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**Contextual**
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```bash
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$ go run ./_examples/shell/help echo --help
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Usage: help echo <msg>
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Regular command help
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🚀 additional command help
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Arguments:
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<msg> Regular argument help
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Flags:
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-h, --help Show context-sensitive help.
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--flag Regular flag help
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```
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#### Showing an _argument_'s detailed help
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Custom help will only be shown for _positional arguments with named fields_ ([see the README section on positional arguments for more details on what that means](../../../README.md#branching-positional-arguments))
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**Contextual argument help**
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```bash
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$ go run ./_examples/shell/help msg --help
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Usage: help echo <msg>
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Regular argument help
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📣 additional argument help
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Flags:
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-h, --help Show context-sensitive help.
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--flag Regular flag help
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```
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## Command handling
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There are two ways to handle commands in Kong.
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### Switch on the command string
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When you call `kong.Parse()` it will return a unique string representation of the command. Each command branch in the hierarchy will be a bare word and each branching argument or required positional argument will be the name surrounded by angle brackets. Here's an example:
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There's an example of this pattern [here](https://github.com/alecthomas/kong/blob/master/_examples/shell/commandstring/main.go).
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eg.
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```go
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package main
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import "github.com/alecthomas/kong"
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var CLI struct {
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Rm struct {
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Force bool `help:"Force removal."`
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Recursive bool `help:"Recursively remove files."`
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Paths []string `arg:"" name:"path" help:"Paths to remove." type:"path"`
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} `cmd:"" help:"Remove files."`
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Ls struct {
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Paths []string `arg:"" optional:"" name:"path" help:"Paths to list." type:"path"`
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} `cmd:"" help:"List paths."`
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}
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func main() {
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ctx := kong.Parse(&CLI)
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switch ctx.Command() {
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case "rm <path>":
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case "ls":
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default:
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panic(ctx.Command())
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}
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}
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```
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This has the advantage that it is convenient, but the downside that if you modify your CLI structure, the strings may change. This can be fragile.
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### Attach a `Run(...) error` method to each command
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A more robust approach is to break each command out into their own structs:
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1. Break leaf commands out into separate structs.
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2. Attach a `Run(...) error` method to all leaf commands.
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3. Call `kong.Kong.Parse()` to obtain a `kong.Context`.
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4. Call `kong.Context.Run(bindings...)` to call the selected parsed command.
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Once a command node is selected by Kong it will search from that node back to the root. Each
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encountered command node with a `Run(...) error` will be called in reverse order. This allows
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sub-trees to be re-used fairly conveniently.
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In addition to values bound with the `kong.Bind(...)` option, any values
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passed through to `kong.Context.Run(...)` are also bindable to the target's
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`Run()` arguments.
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Finally, hooks can also contribute bindings via `kong.Context.Bind()` and `kong.Context.BindTo()`.
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There's a full example emulating part of the Docker CLI [here](https://github.com/alecthomas/kong/tree/master/_examples/docker).
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eg.
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```go
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type Context struct {
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Debug bool
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}
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type RmCmd struct {
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Force bool `help:"Force removal."`
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Recursive bool `help:"Recursively remove files."`
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Paths []string `arg:"" name:"path" help:"Paths to remove." type:"path"`
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}
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func (r *RmCmd) Run(ctx *Context) error {
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fmt.Println("rm", r.Paths)
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return nil
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}
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type LsCmd struct {
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Paths []string `arg:"" optional:"" name:"path" help:"Paths to list." type:"path"`
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}
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func (l *LsCmd) Run(ctx *Context) error {
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fmt.Println("ls", l.Paths)
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return nil
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}
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var cli struct {
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Debug bool `help:"Enable debug mode."`
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Rm RmCmd `cmd:"" help:"Remove files."`
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Ls LsCmd `cmd:"" help:"List paths."`
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}
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func main() {
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ctx := kong.Parse(&cli)
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// Call the Run() method of the selected parsed command.
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err := ctx.Run(&Context{Debug: cli.Debug})
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ctx.FatalIfErrorf(err)
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}
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```
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## Hooks: BeforeReset(), BeforeResolve(), BeforeApply(), AfterApply() and the Bind() option
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If a node in the grammar has a `BeforeReset(...)`, `BeforeResolve
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(...)`, `BeforeApply(...) error` and/or `AfterApply(...) error` method, those
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methods will be called before values are reset, before validation/assignment,
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and after validation/assignment, respectively.
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The `--help` flag is implemented with a `BeforeReset` hook.
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Arguments to hooks are provided via the `Run(...)` method or `Bind(...)` option. `*Kong`, `*Context` and `*Path` are also bound and finally, hooks can also contribute bindings via `kong.Context.Bind()` and `kong.Context.BindTo()`.
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eg.
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```go
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// A flag with a hook that, if triggered, will set the debug loggers output to stdout.
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type debugFlag bool
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func (d debugFlag) BeforeApply(logger *log.Logger) error {
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logger.SetOutput(os.Stdout)
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return nil
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}
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var cli struct {
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Debug debugFlag `help:"Enable debug logging."`
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}
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func main() {
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// Debug logger going to discard.
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logger := log.New(io.Discard, "", log.LstdFlags)
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ctx := kong.Parse(&cli, kong.Bind(logger))
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// ...
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}
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```
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Another example of using hooks is load the env-file:
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```go
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package main
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import (
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"fmt"
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"github.com/alecthomas/kong"
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"github.com/joho/godotenv"
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)
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type EnvFlag string
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// BeforeResolve loads env file.
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func (c EnvFlag) BeforeReset(ctx *kong.Context, trace *kong.Path) error {
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path := string(ctx.FlagValue(trace.Flag).(EnvFlag)) // nolint
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path = kong.ExpandPath(path)
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if err := godotenv.Load(path); err != nil {
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return err
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}
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return nil
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}
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var CLI struct {
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EnvFile EnvFlag
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Flag `env:"FLAG"`
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}
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func main() {
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_ = kong.Parse(&CLI)
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fmt.Println(CLI.Flag)
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}
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```
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## Flags
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Any [mapped](#mapper---customising-how-the-command-line-is-mapped-to-go-values) field in the command structure _not_ tagged with `cmd` or `arg` will be a flag. Flags are optional by default.
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eg. The command-line `app [--flag="foo"]` can be represented by the following.
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```go
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type CLI struct {
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Flag string
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}
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```
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## Commands and sub-commands
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Sub-commands are specified by tagging a struct field with `cmd`. Kong supports arbitrarily nested commands.
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eg. The following struct represents the CLI structure `command [--flag="str"] sub-command`.
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```go
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type CLI struct {
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Command struct {
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Flag string
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SubCommand struct {
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} `cmd`
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} `cmd`
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}
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```
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If a sub-command is tagged with `default:"1"` it will be selected if there are no further arguments. If a sub-command is tagged with `default:"withargs"` it will be selected even if there are further arguments or flags and those arguments or flags are valid for the sub-command. This allows the user to omit the sub-command name on the CLI if its arguments/flags are not ambiguous with the sibling commands or flags.
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## Branching positional arguments
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In addition to sub-commands, structs can also be configured as branching positional arguments.
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This is achieved by tagging an [unmapped](#mapper---customising-how-the-command-line-is-mapped-to-go-values) nested struct field with `arg`, then including a positional argument field inside that struct _with the same name_. For example, the following command structure:
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app rename <name> to <name>
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Can be represented with the following:
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```go
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var CLI struct {
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Rename struct {
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Name struct {
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Name string `arg` // <-- NOTE: identical name to enclosing struct field.
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To struct {
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Name struct {
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Name string `arg`
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} `arg`
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} `cmd`
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} `arg`
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} `cmd`
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}
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```
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This looks a little verbose in this contrived example, but typically this will not be the case.
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## Positional arguments
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If a field is tagged with `arg:""` it will be treated as the final positional
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value to be parsed on the command line. By default positional arguments are
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required, but specifying `optional:""` will alter this.
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If a positional argument is a slice, all remaining arguments will be appended
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to that slice.
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## Slices
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Slice values are treated specially. First the input is split on the `sep:"<rune>"` tag (defaults to `,`), then each element is parsed by the slice element type and appended to the slice. If the same value is encountered multiple times, elements continue to be appended.
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To represent the following command-line:
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cmd ls <file> <file> ...
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You would use the following:
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```go
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var CLI struct {
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Ls struct {
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Files []string `arg:"" type:"existingfile"`
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} `cmd`
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}
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```
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## Maps
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Maps are similar to slices except that only one key/value pair can be assigned per value, and the `sep` tag denotes the assignment character and defaults to `=`.
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To represent the following command-line:
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cmd config set <key>=<value> <key>=<value> ...
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You would use the following:
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```go
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var CLI struct {
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Config struct {
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Set struct {
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Config map[string]float64 `arg:"" type:"file:"`
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} `cmd`
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} `cmd`
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}
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```
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For flags, multiple key+value pairs should be separated by `mapsep:"rune"` tag (defaults to `;`) eg. `--set="key1=value1;key2=value2"`.
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## Pointers
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Pointers work like the underlying type, except that you can differentiate between the presence of the zero value and no value being supplied.
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For example:
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```go
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var CLI struct {
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Foo *int
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}
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```
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Would produce a nil value for `Foo` if no `--foo` argument is supplied, but would have a pointer to the value 0 if the argument `--foo=0` was supplied.
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## Nested data structure
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Kong support a nested data structure as well with `embed:""`. You can combine `embed:""` with `prefix:""`:
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```go
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var CLI struct {
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Logging struct {
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Level string `enum:"debug,info,warn,error" default:"info"`
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Type string `enum:"json,console" default:"console"`
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} `embed:"" prefix:"logging."`
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}
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```
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This configures Kong to accept flags `--logging.level` and `--logging.type`.
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## Custom named decoders
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Kong includes a number of builtin custom type mappers. These can be used by
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specifying the tag `type:"<type>"`. They are registered with the option
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function `NamedMapper(name, mapper)`.
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| Name | Description |
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| -------------- | ---------------------------------------------------------------------------------------------------------------------- |
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| `path` | A path. ~ expansion is applied. `-` is accepted for stdout, and will be passed unaltered. |
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| `existingfile` | An existing file. ~ expansion is applied. `-` is accepted for stdin, and will be passed unaltered. |
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| `existingdir` | An existing directory. ~ expansion is applied. |
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| `counter` | Increment a numeric field. Useful for `-vvv`. Can accept `-s`, `--long` or `--long=N`. |
|
|
| `filecontent` | Read the file at path into the field. ~ expansion is applied. `-` is accepted for stdin, and will be passed unaltered. |
|
|
|
|
Slices and maps treat type tags specially. For slices, the `type:""` tag
|
|
specifies the element type. For maps, the tag has the format
|
|
`tag:"[<key>]:[<value>]"` where either may be omitted.
|
|
|
|
## Supported field types
|
|
|
|
## Custom decoders (mappers)
|
|
|
|
Any field implementing `encoding.TextUnmarshaler` or `json.Unmarshaler` will use those interfaces
|
|
for decoding values. Kong also includes builtin support for many common Go types:
|
|
|
|
| Type | Description |
|
|
| --------------- | ----------------------------------------------------------------------------------------------------------- |
|
|
| `time.Duration` | Populated using `time.ParseDuration()`. |
|
|
| `time.Time` | Populated using `time.Parse()`. Format defaults to RFC3339 but can be overridden with the `format:"X"` tag. |
|
|
| `*os.File` | Path to a file that will be opened, or `-` for `os.Stdin`. File must be closed by the user. |
|
|
| `*url.URL` | Populated with `url.Parse()`. |
|
|
|
|
For more fine-grained control, if a field implements the
|
|
[MapperValue](https://godoc.org/github.com/alecthomas/kong#MapperValue)
|
|
interface it will be used to decode arguments into the field.
|
|
|
|
## Supported tags
|
|
|
|
Tags can be in two forms:
|
|
|
|
1. Standard Go syntax, eg. `kong:"required,name='foo'"`.
|
|
2. Bare tags, eg. `required:"" name:"foo"`
|
|
|
|
Both can coexist with standard Tag parsing.
|
|
|
|
| Tag | Description |
|
|
| -------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
|
|
| `cmd:""` | If present, struct is a command. |
|
|
| `arg:""` | If present, field is an argument. Required by default. |
|
|
| `env:"X,Y,..."` | Specify envars to use for default value. The envs are resolved in the declared order. The first value found is used. |
|
|
| `name:"X"` | Long name, for overriding field name. |
|
|
| `help:"X"` | Help text. |
|
|
| `type:"X"` | Specify [named types](#custom-named-decoders) to use. |
|
|
| `placeholder:"X"` | Placeholder input, if flag. e.g. `` `placeholder:"<the-placeholder>"` `` will show `--flag-name=<the-placeholder>` when displaying help. |
|
|
| `default:"X"` | Default value. |
|
|
| `default:"1"` | On a command, make it the default. |
|
|
| `default:"withargs"` | On a command, make it the default and allow args/flags from that command |
|
|
| `short:"X"` | Short name, if flag. |
|
|
| `aliases:"X,Y"` | One or more aliases (for cmd or flag). |
|
|
| `required:""` | If present, flag/arg is required. |
|
|
| `optional:""` | If present, flag/arg is optional. |
|
|
| `hidden:""` | If present, command or flag is hidden. |
|
|
| `negatable:""` | If present on a `bool` field, supports prefixing a flag with `--no-` to invert the default value |
|
|
| `negatable:"X"` | If present on a `bool` field, supports `--X` to invert the default value |
|
|
| `format:"X"` | Format for parsing input, if supported. |
|
|
| `sep:"X"` | Separator for sequences (defaults to ","). May be `none` to disable splitting. |
|
|
| `mapsep:"X"` | Separator for maps (defaults to ";"). May be `none` to disable splitting. |
|
|
| `enum:"X,Y,..."` | Set of valid values allowed for this flag. An enum field must be `required` or have a valid `default`. |
|
|
| `group:"X"` | Logical group for a flag or command. |
|
|
| `xor:"X,Y,..."` | Exclusive OR groups for flags. Only one flag in the group can be used which is restricted within the same command. When combined with `required`, at least one of the `xor` group will be required. |
|
|
| `and:"X,Y,..."` | AND groups for flags. All flags in the group must be used in the same command. When combined with `required`, all flags in the group will be required. |
|
|
| `prefix:"X"` | Prefix for all sub-flags. |
|
|
| `envprefix:"X"` | Envar prefix for all sub-flags. |
|
|
| `set:"K=V"` | Set a variable for expansion by child elements. Multiples can occur. |
|
|
| `embed:""` | If present, this field's children will be embedded in the parent. Useful for composition. |
|
|
| `passthrough:"<mode>"`[^1] | If present on a positional argument, it stops flag parsing when encountered, as if `--` was processed before. Useful for external command wrappers, like `exec`. On a command it requires that the command contains only one argument of type `[]string` which is then filled with everything following the command, unparsed. |
|
|
| `-` | Ignore the field. Useful for adding non-CLI fields to a configuration struct. e.g `` `kong:"-"` `` |
|
|
|
|
[^1]: `<mode>` can be `partial` or `all` (the default). `all` will pass through all arguments including flags, including
|
|
flags. `partial` will validate flags until the first positional argument is encountered, then pass through all remaining
|
|
positional arguments.
|
|
|
|
## Plugins
|
|
|
|
Kong CLI's can be extended by embedding the `kong.Plugin` type and populating it with pointers to Kong annotated structs. For example:
|
|
|
|
```go
|
|
var pluginOne struct {
|
|
PluginOneFlag string
|
|
}
|
|
var pluginTwo struct {
|
|
PluginTwoFlag string
|
|
}
|
|
var cli struct {
|
|
BaseFlag string
|
|
kong.Plugins
|
|
}
|
|
cli.Plugins = kong.Plugins{&pluginOne, &pluginTwo}
|
|
```
|
|
|
|
Additionally if an interface type is embedded, it can also be populated with a Kong annotated struct.
|
|
|
|
## Dynamic Commands
|
|
|
|
While plugins give complete control over extending command-line interfaces, Kong
|
|
also supports dynamically adding commands via `kong.DynamicCommand()`.
|
|
|
|
## Variable interpolation
|
|
|
|
Kong supports limited variable interpolation into help strings, enum lists and
|
|
default values.
|
|
|
|
Variables are in the form:
|
|
|
|
${<name>}
|
|
${<name>=<default>}
|
|
|
|
Variables are set with the `Vars{"key": "value", ...}` option. Undefined
|
|
variable references in the grammar without a default will result in an error at
|
|
construction time.
|
|
|
|
Variables can also be set via the `set:"K=V"` tag. In this case, those variables will be available for that
|
|
node and all children. This is useful for composition by allowing the same struct to be reused.
|
|
|
|
When interpolating into flag or argument help strings, some extra variables
|
|
are defined from the value itself:
|
|
|
|
${default}
|
|
${enum}
|
|
|
|
For flags with associated environment variables, the variable `${env}` can be
|
|
interpolated into the help string. In the absence of this variable in the
|
|
help string, Kong will append `($$${env})` to the help string.
|
|
|
|
eg.
|
|
|
|
```go
|
|
type cli struct {
|
|
Config string `type:"path" default:"${config_file}"`
|
|
}
|
|
|
|
func main() {
|
|
kong.Parse(&cli,
|
|
kong.Vars{
|
|
"config_file": "~/.app.conf",
|
|
})
|
|
}
|
|
```
|
|
|
|
## Validation
|
|
|
|
Kong does validation on the structure of a command-line, but also supports
|
|
extensible validation. Any node in the tree may implement either of the following interfaces:
|
|
|
|
```go
|
|
type Validatable interface {
|
|
Validate() error
|
|
}
|
|
```
|
|
|
|
```go
|
|
type Validatable interface {
|
|
Validate(kctx *kong.Context) error
|
|
}
|
|
```
|
|
|
|
If one of these nodes is in the active command-line it will be called during
|
|
normal validation.
|
|
|
|
## Modifying Kong's behaviour
|
|
|
|
Each Kong parser can be configured via functional options passed to `New(cli interface{}, options...Option)`.
|
|
|
|
The full set of options can be found [here](https://godoc.org/github.com/alecthomas/kong#Option).
|
|
|
|
### `Name(help)` and `Description(help)` - set the application name description
|
|
|
|
Set the application name and/or description.
|
|
|
|
The name of the application will default to the binary name, but can be overridden with `Name(name)`.
|
|
|
|
As with all help in Kong, text will be wrapped to the terminal.
|
|
|
|
### `Configuration(loader, paths...)` - load defaults from configuration files
|
|
|
|
This option provides Kong with support for loading defaults from a set of configuration files. Each file is opened, if possible, and the loader called to create a resolver for that file.
|
|
|
|
eg.
|
|
|
|
```go
|
|
kong.Parse(&cli, kong.Configuration(kong.JSON, "/etc/myapp.json", "~/.myapp.json"))
|
|
```
|
|
|
|
[See the tests](https://github.com/alecthomas/kong/blob/master/resolver_test.go#L206) for an example of how the JSON file is structured.
|
|
|
|
#### List of Configuration Loaders
|
|
|
|
- [YAML](https://github.com/alecthomas/kong-yaml)
|
|
- [HCL](https://github.com/alecthomas/kong-hcl)
|
|
- [TOML](https://github.com/alecthomas/kong-toml)
|
|
- [JSON](https://github.com/alecthomas/kong)
|
|
|
|
### `Resolver(...)` - support for default values from external sources
|
|
|
|
Resolvers are Kong's extension point for providing default values from external sources. As an example, support for environment variables via the `env` tag is provided by a resolver. There's also a builtin resolver for JSON configuration files.
|
|
|
|
Example resolvers can be found in [resolver.go](https://github.com/alecthomas/kong/blob/master/resolver.go).
|
|
|
|
### `*Mapper(...)` - customising how the command-line is mapped to Go values
|
|
|
|
Command-line arguments are mapped to Go values via the Mapper interface:
|
|
|
|
```go
|
|
// A Mapper represents how a field is mapped from command-line values to Go.
|
|
//
|
|
// Mappers can be associated with concrete fields via pointer, reflect.Type, reflect.Kind, or via a "type" tag.
|
|
//
|
|
// Additionally, if a type implements the MapperValue interface, it will be used.
|
|
type Mapper interface {
|
|
// Decode ctx.Value with ctx.Scanner into target.
|
|
Decode(ctx *DecodeContext, target reflect.Value) error
|
|
}
|
|
```
|
|
|
|
All builtin Go types (as well as a bunch of useful stdlib types like `time.Time`) have mappers registered by default. Mappers for custom types can be added using `kong.??Mapper(...)` options. Mappers are applied to fields in four ways:
|
|
|
|
1. `NamedMapper(string, Mapper)` and using the tag key `type:"<name>"`.
|
|
2. `KindMapper(reflect.Kind, Mapper)`.
|
|
3. `TypeMapper(reflect.Type, Mapper)`.
|
|
4. `ValueMapper(interface{}, Mapper)`, passing in a pointer to a field of the grammar.
|
|
|
|
### `ConfigureHelp(HelpOptions)` and `Help(HelpFunc)` - customising help
|
|
|
|
The default help output is usually sufficient, but if not there are two solutions.
|
|
|
|
1. Use `ConfigureHelp(HelpOptions)` to configure how help is formatted (see [HelpOptions](https://godoc.org/github.com/alecthomas/kong#HelpOptions) for details).
|
|
2. Custom help can be wired into Kong via the `Help(HelpFunc)` option. The `HelpFunc` is passed a `Context`, which contains the parsed context for the current command-line. See the implementation of `DefaultHelpPrinter` for an example.
|
|
3. Use `ValueFormatter(HelpValueFormatter)` if you want to just customize the help text that is accompanied by flags and arguments.
|
|
4. Use `Groups([]Group)` if you want to customize group titles or add a header.
|
|
|
|
### Injecting values into `Run()` methods
|
|
|
|
There are several ways to inject values into `Run()` methods:
|
|
|
|
1. Use `Bind()` to bind values directly.
|
|
2. Use `BindTo()` to bind values to an interface type.
|
|
3. Use `BindToProvider()` to bind values to a function that provides the value.
|
|
4. Implement `Provide<Type>() error` methods on the command structure.
|
|
|
|
### Other options
|
|
|
|
The full set of options can be found [here](https://godoc.org/github.com/alecthomas/kong#Option).
|