Add support for plugins.

README.md

@@ -4,34 +4,36 @@
 # Kong is a command-line parser for Go
 [![](https://godoc.org/github.com/alecthomas/kong?status.svg)](http://godoc.org/github.com/alecthomas/kong) [![CircleCI](https://img.shields.io/circleci/project/github/alecthomas/kong.svg)](https://circleci.com/gh/alecthomas/kong) [![Go Report Card](https://goreportcard.com/badge/github.com/alecthomas/kong)](https://goreportcard.com/report/github.com/alecthomas/kong) [![Slack chat](https://img.shields.io/static/v1?logo=slack&style=flat&label=slack&color=green&message=gophers)](https://gophers.slack.com/messages/CN9DS8YF3)
 
-[TOC levels=2-3 numbered]: # "#### Table of Contents"
+<!-- TOC depthFrom:2 updateOnSave:true withLinks:true -->
 
-#### Table of Contents
+- [Introduction](#introduction)
-1. [Introduction](#introduction)
+- [Help](#help)
-1. [Help](#help)
+- [Command handling](#command-handling)
-1. [Command handling](#command-handling)
+    - [Switch on the command string](#switch-on-the-command-string)
-   1. [Switch on the command string](#switch-on-the-command-string)
+    - [Attach a `Run(...) error` method to each command](#attach-a-run-error-method-to-each-command)
-   1. [Attach a `Run(...) error` method to each command](#attach-a-run-error-method-to-each-command)
+- [Hooks: BeforeResolve(), BeforeApply(), AfterApply() and the Bind() option](#hooks-beforeresolve-beforeapply-afterapply-and-the-bind-option)
-1. [Hooks: BeforeResolve(), BeforeApply(), AfterApply() and the Bind() option](#hooks-beforeresolve-beforeapply-afterapply-and-the-bind-option)
+- [Flags](#flags)
-1. [Flags](#flags)
+- [Commands and sub-commands](#commands-and-sub-commands)
-1. [Commands and sub-commands](#commands-and-sub-commands)
+- [Branching positional arguments](#branching-positional-arguments)
-1. [Branching positional arguments](#branching-positional-arguments)
+- [Terminating positional arguments](#terminating-positional-arguments)
-1. [Terminating positional arguments](#terminating-positional-arguments)
+- [Slices](#slices)
-1. [Slices](#slices)
+- [Maps](#maps)
-1. [Maps](#maps)
+- [Custom named decoders](#custom-named-decoders)
-1. [Custom named decoders](#custom-named-decoders)
+- [Supported field types](#supported-field-types)
-1. [Custom decoders (mappers)](#custom-decoders-mappers)
+- [Custom decoders (mappers)](#custom-decoders-mappers)
-1. [Supported tags](#supported-tags)
+- [Supported tags](#supported-tags)
-1. [Variable interpolation](#variable-interpolation)
+- [Plugins](#plugins)
-1. [Modifying Kong's behaviour](#modifying-kongs-behaviour)
+- [Variable interpolation](#variable-interpolation)
-   1. [`Name(help)` and `Description(help)` - set the application name description](#namehelp-and-descriptionhelp---set-the-application-name-description)
+- [Modifying Kong's behaviour](#modifying-kongs-behaviour)
-   1. [`Configuration(loader, paths...)` - load defaults from configuration files](#configurationloader-paths---load-defaults-from-configuration-files)
+    - [`Name(help)` and `Description(help)` - set the application name description](#namehelp-and-descriptionhelp---set-the-application-name-description)
-   1. [`Resolver(...)` - support for default values from external sources](#resolver---support-for-default-values-from-external-sources)
+    - [`Configuration(loader, paths...)` - load defaults from configuration files](#configurationloader-paths---load-defaults-from-configuration-files)
-   1. [`*Mapper(...)` - customising how the command-line is mapped to Go values](#mapper---customising-how-the-command-line-is-mapped-to-go-values)
+    - [`Resolver(...)` - support for default values from external sources](#resolver---support-for-default-values-from-external-sources)
-   1. [`ConfigureHelp(HelpOptions)` and `Help(HelpFunc)` - customising help](#configurehelphelpoptions-and-helphelpfunc---customising-help)
+    - [`*Mapper(...)` - customising how the command-line is mapped to Go values](#mapper---customising-how-the-command-line-is-mapped-to-go-values)
-   1. [`Bind(...)` - bind values for callback hooks and Run() methods](#bind---bind-values-for-callback-hooks-and-run-methods)
+    - [`ConfigureHelp(HelpOptions)` and `Help(HelpFunc)` - customising help](#configurehelphelpoptions-and-helphelpfunc---customising-help)
-   1. [Other options](#other-options)
+    - [`Bind(...)` - bind values for callback hooks and Run() methods](#bind---bind-values-for-callback-hooks-and-run-methods)
+    - [Other options](#other-options)
 
+<!-- /TOC -->
 
 ## Introduction
 
@@ -437,6 +439,26 @@ Tag                    | Description
 `embed`                | If present, this field's children will be embedded in the parent. Useful for composition.
 `-`                    | Ignore the field. Useful for adding non-CLI fields to a configuration struct.
 
+## 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.
+
 ## Variable interpolation
 
 Kong supports limited variable interpolation into help strings, enum lists and

build.go

@@ -6,6 +6,11 @@ import (
 	"strings"
 )
 
+// Plugins are dynamically embedded command-line structures.
+//
+// Each element in the Plugins list *must* be a pointer to a structure.
+type Plugins []interface{}
+
 func build(k *Kong, ast interface{}) (app *Application, err error) {
 	defer catch(&err)
 	v := reflect.ValueOf(ast)
@@ -50,28 +55,34 @@ func flattenedFields(v reflect.Value) (out []flattenedField) {
 		if tag.Ignored {
 			continue
 		}
-		if ft.Anonymous || tag.Embed {
+		if !ft.Anonymous && !tag.Embed {
-			if fv.Kind() == reflect.Interface {
+			if fv.CanSet() {
-				fv = fv.Elem()
+				out = append(out, flattenedField{field: ft, value: fv, tag: tag})
 			}
-			sub := flattenedFields(fv)
-			for _, subf := range sub {
-				// Assign parent if it's not already set.
-				if subf.tag.Group == "" {
-					subf.tag.Group = tag.Group
-				}
-				// Accumulate prefixes.
-				subf.tag.Prefix = tag.Prefix + subf.tag.Prefix
-				// Combine parent vars.
-				subf.tag.Vars = tag.Vars.CloneWith(subf.tag.Vars)
-			}
-			out = append(out, sub...)
 			continue
 		}
-		if !fv.CanSet() {
+
+		// Embedded type.
+		if fv.Kind() == reflect.Interface {
+			fv = fv.Elem()
+		} else if fv.Type() == reflect.TypeOf(Plugins{}) {
+			for i := 0; i < fv.Len(); i++ {
+				out = append(out, flattenedFields(fv.Index(i).Elem())...)
+			}
 			continue
 		}
-		out = append(out, flattenedField{field: ft, value: fv, tag: tag})
+		sub := flattenedFields(fv)
+		for _, subf := range sub {
+			// Assign parent if it's not already set.
+			if subf.tag.Group == "" {
+				subf.tag.Group = tag.Group
+			}
+			// Accumulate prefixes.
+			subf.tag.Prefix = tag.Prefix + subf.tag.Prefix
+			// Combine parent vars.
+			subf.tag.Vars = tag.Vars.CloneWith(subf.tag.Vars)
+		}
+		out = append(out, sub...)
 	}
 	return out
 }

kong_test.go

@@ -866,3 +866,24 @@ func TestLoneHpyhen(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, "-", cli.Flag)
 }
+
+func TestPlugins(t *testing.T) {
+	var pluginOne struct {
+		One string
+	}
+	var pluginTwo struct {
+		Two string
+	}
+	var cli struct {
+		Base string
+		kong.Plugins
+	}
+	cli.Plugins = kong.Plugins{&pluginOne, &pluginTwo}
+
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--base=base", "--one=one", "--two=two"})
+	require.NoError(t, err)
+	require.Equal(t, "base", cli.Base)
+	require.Equal(t, "one", pluginOne.One)
+	require.Equal(t, "two", pluginTwo.Two)
+}