feat: allow non-structs to be used as commands (#428)

* feat: allow non-structs to be used as commands

This small MR allows using the func-to-interface trick to implement a command (see commandFunc in kong_test.go).

This is useful e.g. for commands that have no flags or arguments of their own, but instead receive all required information via bound  parameters.

* fix: check DynamicCommand is runnable when adding

build.go

@@ -51,6 +51,9 @@ type flattenedField struct {
 
 func flattenedFields(v reflect.Value, ptag *Tag) (out []flattenedField, err error) {
 	v = reflect.Indirect(v)
+	if v.Kind() != reflect.Struct {
+		return out, nil
+	}
 	for i := 0; i < v.NumField(); i++ {
 		ft := v.Type().Field(i)
 		fv := v.Field(i)

context.go

@@ -903,7 +903,7 @@ func checkMissingChildren(node *Node) error {
 	if len(missing) == 1 {
 		return fmt.Errorf("expected %s", missing[0])
 	}
-	return fmt.Errorf("expected one of %s", strings.Join(missing, ",  "))
+	return fmt.Errorf("expected one of %s", strings.Join(missing, ", "))
 }
 
 // If we're missing any positionals and they're required, return an error.

kong_test.go

@@ -1297,6 +1297,12 @@ func (d *dynamicCommand) Run() error {
 	return nil
 }
 
+type commandFunc func() error
+
+func (cf commandFunc) Run() error {
+	return cf()
+}
+
 func TestDynamicCommands(t *testing.T) {
 	cli := struct {
 		One struct{} `cmd:"one"`
@@ -1304,9 +1310,12 @@ func TestDynamicCommands(t *testing.T) {
 	help := &strings.Builder{}
 	two := &dynamicCommand{}
 	three := &dynamicCommand{}
+	fourRan := false
+	four := commandFunc(func() error { fourRan = true; return nil })
 	p := mustNew(t, &cli,
 		kong.DynamicCommand("two", "", "", &two),
 		kong.DynamicCommand("three", "", "", three, "hidden"),
+		kong.DynamicCommand("four", "", "", &four),
 		kong.Writers(help, help),
 		kong.Exit(func(int) {}))
 	kctx, err := p.Parse([]string{"two", "--flag=flag"})
@@ -1317,8 +1326,15 @@ func TestDynamicCommands(t *testing.T) {
 	assert.NoError(t, err)
 	assert.True(t, two.ran)
 
+	kctx, err = p.Parse([]string{"four"})
+	assert.NoError(t, err)
+	assert.False(t, fourRan)
+	err = kctx.Run()
+	assert.NoError(t, err)
+	assert.True(t, fourRan)
+
 	_, err = p.Parse([]string{"--help"})
-	assert.EqualError(t, err, `expected one of "one",  "two"`)
+	assert.EqualError(t, err, `expected one of "one", "two", "four"`)
 	assert.NotContains(t, help.String(), "three", help.String())
 }
 

options.go

@@ -89,6 +89,10 @@ type dynamicCommand struct {
 // "tags" is a list of extra tag strings to parse, in the form <key>:"<value>".
 func DynamicCommand(name, help, group string, cmd interface{}, tags ...string) Option {
 	return OptionFunc(func(k *Kong) error {
+		if run := getMethod(reflect.Indirect(reflect.ValueOf(cmd)), "Run"); !run.IsValid() {
+			return fmt.Errorf("kong: DynamicCommand %q must be a type with a 'Run' method; got %T", name, cmd)
+		}
+
 		k.dynamicCommands = append(k.dynamicCommands, &dynamicCommand{
 			name:  name,
 			help:  help,