Alec Thomas
210 lines
6.5 KiB
Markdown
210 lines
6.5 KiB
Markdown
# Kong is a command-line parser for Go [](https://circleci.com/gh/alecthomas/kong)
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<!-- MarkdownTOC -->
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1. [Introduction](#introduction)
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1. [Help](#help)
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1. [Flags](#flags)
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1. [Commands and sub-commands](#commands-and-sub-commands)
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1. [Supported tags](#supported-tags)
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1. [Configuring Kong](#configuring-kong)
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1. [`*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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1. [`Help(HelpFunc)` - customising help](#helphelpfunc---customising-help)
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1. [`Hook(&field, HookFunc)` - callback hooks to execute when the command-line is parsed](#hookfield-hookfunc---callback-hooks-to-execute-when-the-command-line-is-parsed)
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<!-- /MarkdownTOC -->
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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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```
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shell rm [-f] [-r] <paths> ...
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shell ls [<paths> ...]
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```
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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 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 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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kong.Parse(&CLI)
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}
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```
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## Help
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Help is automatically generated. With no other arguments provided, help will display a full summary of all available commands.
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eg.
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```
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$ shell --help
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usage: shell [<flags>]
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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 [<flags>] <paths> ...
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Remove files.
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ls [<flags>] [<paths> ...]
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List paths.
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```
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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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```
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$ shell --help rm
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usage: shell rm [<flags>] <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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```
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## Flags
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Any field in the command structure *not* tagged with `cmd` or `arg` will be a flag. Flags are optional by default.
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## Commands and sub-commands
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Kong supports arbitrarily nested commands and positional arguments. Nested structs tagged with `cmd` will be treated as commands.
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Arguments can also optionally have children, in order to support commands like the following:
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```
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app rename <name> to <name>
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```
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This is achieved by tagging a nested struct with `arg`, then including a positional argument field inside that struct with the same name. For example:
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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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## Supported tags
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Tags can be in two forms:
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1. Standard Go syntax, eg. `kong:"required,name='foo'"`.
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2. Bare tags, eg. `required name:"foo"`
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Both can coexist with standard Tag parsing.
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| Tag | Description |
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| -----------------------| ------------------------------------------- |
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| `cmd` | If present, struct is a command. |
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| `arg` | If present, field is an argument. |
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| `type:"X"` | Specify named Mapper to use. |
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| `help:"X"` | Help text. |
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| `placeholder:"X"` | Placeholder text. |
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| `default:"X"` | Default value. |
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| `short:"X"` | Short name, if flag. |
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| `name:"X"` | Long name, for overriding field name. |
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| `required` | If present, flag/arg is required. |
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| `optional` | If present, flag/arg is optional. |
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| `hidden` | If present, flag is hidden. |
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| `format:"X"` | Format for parsing input, if supported. |
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| `sep:"X"` | Separator for sequences (defaults to ",") |
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## Configuring Kong
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Each Kong parser can be configured via functional options passed to `New(cli interface{}, options...Option)`. The full set of options can be found in `options.go`.
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### `*Mapper(...)` - customising how the command-line is mapped to Go values
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Command-line arguments are mapped to Go values via the Mapper interface:
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```go
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// A Mapper knows how to map command-line input to Go.
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type Mapper interface {
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// Decode scan into target.
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//
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// "ctx" contains context about the value being decoded that may be useful
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// to some mapperss.
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Decode(ctx *MapperContext, scan *Scanner, target reflect.Value) error
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}
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```
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All builtin Go types (as well as a bunch of useful stdlib types like `time.Time`) have mapperss registered by default. Mappers for custom types can be added using `kong.??Mapper(...)` options. Mappers are applied to fields in four ways:
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1. `NamedMapper(string, Mapper)` and using the tag key `type:"<name>"`.
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2. `KindMapper(reflect.Kind, Mapper)`.
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3. `TypeMapper(reflect.Type, Mapper)`.
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4. `ValueMapper(interface{}, Mapper)`, passing in a pointer to a field of the grammar.
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### `Help(HelpFunc)` - customising help
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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 `PrintHelp` for an example.
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### `Hook(&field, HookFunc)` - callback hooks to execute when the command-line is parsed
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Hooks are callback functions that are bound to a node in the command-line and executed at parse time, before structural validation and assignment.
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eg.
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```go
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app := kong.Must(&CLI, kong.Hook(&CLI.Debug, func(ctx *Context, path *Path) error {
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log.SetLevel(DEBUG)
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return nil
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}))
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```
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Note: it is generally more advisable to use an imperative approach to building command-lines, eg.
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```go
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if CLI.Debug {
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log.SetLevel(DEBUG)
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}
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```
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But under some circumstances, hooks are the right choice.
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