Support detailed help.

Any command/arg implementing the HelpProvider interface will be used to
provide arbitrarily detailed help.

build.go

@@ -6,6 +6,8 @@ import (
 	"strings"
 )
 
+var helpProviderType = reflect.TypeOf((*HelpProvider)(nil)).Elem()
+
 func build(k *Kong, ast interface{}) (app *Application, err error) {
 	defer catch(&err)
 	v := reflect.ValueOf(ast)
@@ -107,6 +109,10 @@ func buildChild(k *Kong, node *Node, typ NodeType, v reflect.Value, ft reflect.S
 	child.Help = tag.Help
 	child.Hidden = tag.Hidden
 
+	if fv.Type().Implements(helpProviderType) {
+		child.Detail = fv.Interface().(HelpProvider).Help()
+	}
+
 	// A branching argument. This is a bit hairy, as we let buildNode() do the parsing, then check that
 	// a positional argument is provided to the child, and move it to the branching argument field.
 	if tag.Arg {

help.go

@@ -25,6 +25,12 @@ type HelpOptions struct {
 	Compact bool
 }
 
+// HelpProvider can be implemented by commands/args to provide detailed help.
+type HelpProvider interface {
+	// This string is formatted by go/doc and thus has the same formatting rules.
+	Help() string
+}
+
 // HelpPrinter is used to print context-sensitive help.
 type HelpPrinter func(options HelpOptions, ctx *Context) error
 
@@ -77,6 +83,10 @@ func printNodeDetail(w *helpWriter, node *Node) {
 	if w.Summary {
 		return
 	}
+	if node.Detail != "" {
+		w.Print("")
+		w.Wrap(node.Detail)
+	}
 	if len(node.Positional) > 0 {
 		w.Print("")
 		w.Print("Arguments:")

help_test.go

@@ -9,6 +9,16 @@ import (
 	"github.com/alecthomas/kong"
 )
 
+// nolint: govet
+type threeArg struct {
+	RequiredThree bool   `required`
+	Three         string `arg`
+}
+
+func (threeArg) Help() string {
+	return `Detailed help provided through the HelpProvider interface.`
+}
+
 func TestHelp(t *testing.T) {
 	// nolint: govet
 	var cli struct {
@@ -26,10 +36,7 @@ func TestHelp(t *testing.T) {
 			Flag        string `help:"Nested flag under two."`
 			RequiredTwo bool   `required`
 
-			Three struct {
-				RequiredThree bool   `required`
-				Three         string `arg`
-			} `arg help:"Sub-sub-arg."`
+			Three threeArg `arg help:"Sub-sub-arg."`
 
 			Four struct {
 			} `cmd help:"Sub-sub-command."`
@@ -95,6 +102,8 @@ Run "test-app <command> --help" for more information on a command.
 
 Sub-sub-arg.
 
+Detailed help provided through the HelpProvider interface.
+
 Flags:
   --help              Show context-sensitive help.
   --string=STRING     A string flag.

model.go

@@ -35,7 +35,8 @@ type Node struct {
 	Type       NodeType
 	Parent     *Node
 	Name       string
-	Help       string
+	Help       string // Short help displayed in summaries.
+	Detail     string // Detailed help displayed when describing command/arg alone.
 	Hidden     bool
 	Flags      []*Flag
 	Positional []*Positional