Convert Hook to an Option.

help.go

@@ -18,7 +18,7 @@ usage: {{.Name}}
 var defaultHelpTemplate = template.Must(template.New("help").Parse(defaultHelp))
 
 // Help returns a Hook that will display help and exit.
-func Help(tmpl *template.Template, tmplctx map[string]interface{}) Hook {
+func Help(tmpl *template.Template, tmplctx map[string]interface{}) HookFunction {
 	return func(app *Kong, ctx *Context, trace *Trace) error {
 		merged := map[string]interface{}{
 			"Application": app.Model,

kong.go

@@ -9,7 +9,7 @@ import (
 	"text/template"
 )
 
-type Hook func(app *Kong, ctx *Context, trace *Trace) error
+type HookFunction func(app *Kong, ctx *Context, trace *Trace) error
 
 // Error reported by Kong.
 type Error struct{ msg string }
@@ -37,10 +37,11 @@ type Kong struct {
 	Stdout io.Writer
 	Stderr io.Writer
 
-	help        *template.Template
-	helpContext map[string]interface{}
-	helpFuncs   template.FuncMap
-	hooks       map[reflect.Value]Hook
+	help          *template.Template
+	helpContext   map[string]interface{}
+	helpFuncs     template.FuncMap
+	hooks         map[reflect.Value]HookFunction
+	noDefaultHelp bool
 }
 
 // New creates a new Kong parser into ast.
@@ -52,7 +53,7 @@ func New(ast interface{}, options ...Option) (*Kong, error) {
 		help:        defaultHelpTemplate,
 		helpContext: map[string]interface{}{},
 		helpFuncs:   template.FuncMap{},
-		hooks:       map[reflect.Value]Hook{},
+		hooks:       map[reflect.Value]HookFunction{},
 	}
 
 	model, err := build(ast)
@@ -66,9 +67,28 @@ func New(ast interface{}, options ...Option) (*Kong, error) {
 		option(k)
 	}
 
+	if !k.noDefaultHelp {
+		k.integrateHelp()
+	}
+
 	return k, nil
 }
 
+func (k *Kong) integrateHelp() {
+	helpValue := false
+	help := &Flag{
+		Value: Value{
+			Name:    "help",
+			Help:    "Show context-sensitive help.",
+			Flag:    true,
+			Value:   reflect.ValueOf(&helpValue).Elem(),
+			Decoder: kindDecoders[reflect.Bool],
+		},
+	}
+	k.Model.Flags = append([]*Flag{help}, k.Model.Flags...)
+	Hook(&helpValue, Help(defaultHelpTemplate, nil))(k)
+}
+
 // Trace parses the command-line, validating and collecting matching grammar nodes.
 func (k *Kong) Trace(args []string) (*Context, error) {
 	p := &Context{
@@ -89,12 +109,6 @@ func (k *Kong) Trace(args []string) (*Context, error) {
 	return p, nil
 }
 
-// Hook to execute when a command is encountered.
-func (k *Kong) Hook(ptr interface{}, hook Hook) *Kong {
-	k.hooks[reflect.ValueOf(ptr)] = hook
-	return k
-}
-
 // Parse arguments into target.
 //
 // The returned "command" is a space separated path to the final selected command, if any. Commands appear as

kong_test.go

@@ -7,11 +7,12 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func mustNew(t *testing.T, cli interface{}) *Kong {
+func mustNew(t *testing.T, cli interface{}, options ...Option) *Kong {
 	t.Helper()
-	parser, err := New(cli, ExitFunction(func(int) {
+	options = append(options, ExitFunction(func(int) {
 		t.Fatalf("unexpected exit()")
 	}))
+	parser, err := New(cli, options...)
 	require.NoError(t, err)
 	return parser
 }
@@ -381,19 +382,13 @@ func TestHooks(t *testing.T) {
 		{"Flag", "one --three=three", values{true, "", "three"}},
 		{"ArgAndFlag", "one two --three=three", values{true, "two", "three"}},
 	}
-	p := mustNew(t, &cli).
-		Hook(&cli.One, func(app *Kong, ctx *Context, trace *Trace) error {
-			hooked.one = true
-			return nil
-		}).
-		Hook(&cli.One.Two, func(app *Kong, ctx *Context, trace *Trace) error {
-			hooked.two = trace.Value.String()
-			return nil
-		}).
-		Hook(&cli.One.Three, func(app *Kong, ctx *Context, trace *Trace) error {
-			hooked.three = trace.Value.String()
-			return nil
-		})
+	setOne := func(app *Kong, ctx *Context, trace *Trace) error { hooked.one = true; return nil }
+	setTwo := func(app *Kong, ctx *Context, trace *Trace) error { hooked.two = trace.Value.String(); return nil }
+	setThree := func(app *Kong, ctx *Context, trace *Trace) error { hooked.three = trace.Value.String(); return nil }
+	p := mustNew(t, &cli,
+		Hook(&cli.One, setOne),
+		Hook(&cli.One.Two, setTwo),
+		Hook(&cli.One.Three, setThree))
 
 	for _, test := range tests {
 		hooked = values{}

options.go

@@ -2,6 +2,7 @@ package kong
 
 import (
 	"io"
+	"reflect"
 	"text/template"
 )
 
@@ -12,6 +13,13 @@ func ExitFunction(exit func(int)) Option {
 	return func(k *Kong) { k.Exit = exit }
 }
 
+// NoDefaultHelp disables the default help flags.
+func NoDefaultHelp() Option {
+	return func(k *Kong) {
+		k.noDefaultHelp = true
+	}
+}
+
 // Name overrides the application name.
 func Name(name string) Option {
 	return func(k *Kong) { k.Model.Name = name }
@@ -41,3 +49,14 @@ func Writers(stdout, stderr io.Writer) Option {
 		k.Stderr = stderr
 	}
 }
+
+// Hook to execute when a command, flag or positional argument is encountered.
+func Hook(ptr interface{}, hook HookFunction) Option {
+	key := reflect.ValueOf(ptr)
+	if key.Kind() != reflect.Ptr {
+		panic("expected a pointer")
+	}
+	return func(k *Kong) {
+		k.hooks[key] = hook
+	}
+}