Add BindTo option for binding interface implementations.

options.go

@@ -126,6 +126,16 @@ func Bind(args ...interface{}) OptionFunc {
 	}
 }
 
+// BindTo allows binding of implementations to interfaces.
+//
+// 		BindTo(impl, (*iface)(nil))
+func BindTo(impl, iface interface{}) OptionFunc {
+	return func(k *Kong) error {
+		k.bindings[reflect.TypeOf(iface).Elem()] = reflect.ValueOf(impl)
+		return nil
+	}
+}
+
 // Help printer to use.
 func Help(help HelpPrinter) OptionFunc {
 	return func(k *Kong) error {

options_test.go

@@ -1,19 +1,18 @@
-package kong_test
+package kong
 
 import (
 	"encoding/json"
 	"io/ioutil"
 	"os"
+	"reflect"
 	"testing"
 
 	"github.com/stretchr/testify/require"
-
-	"github.com/alecthomas/kong"
 )
 
 func TestOptions(t *testing.T) {
 	var cli struct{}
-	p, err := kong.New(&cli, kong.Name("name"), kong.Description("description"), kong.Writers(nil, nil), kong.Exit(nil))
+	p, err := New(&cli, Name("name"), Description("description"), Writers(nil, nil), Exit(nil))
 	require.NoError(t, err)
 	require.Equal(t, "name", p.Model.Name)
 	require.Equal(t, "description", p.Model.Help)
@@ -44,8 +43,33 @@ func TestConfigLoading(t *testing.T) {
 	err = json.NewEncoder(second).Encode(&cli)
 	require.NoError(t, err)
 
-	p := mustNew(t, &cli, kong.Configuration(kong.JSON, first.Name(), second.Name()))
+	p, err := New(&cli, Configuration(JSON, first.Name(), second.Name()))
+	require.NoError(t, err)
 	_, err = p.Parse(nil)
 	require.NoError(t, err)
 	require.Equal(t, "first", cli.Flag)
 }
+
+type impl string
+
+func (impl) Method() {}
+
+func TestBindTo(t *testing.T) {
+	type iface interface {
+		Method()
+	}
+
+	saw := ""
+	method := func(i iface) error { // nolint: unparam
+		saw = string(i.(impl))
+		return nil
+	}
+
+	var cli struct{}
+
+	p, err := New(&cli, BindTo(impl("foo"), (*iface)(nil)))
+	require.NoError(t, err)
+	err = callMethod("method", reflect.ValueOf(impl("??")), reflect.ValueOf(method), p.bindings)
+	require.NoError(t, err)
+	require.Equal(t, "foo", saw)
+}