Move .Set = true into Decode().

build.go

@@ -110,6 +110,7 @@ func buildNode(v reflect.Value, cmd bool) *Node {
 				Name:    name,
 				Flag:    flag,
 				Help:    help,
+				Default: dflt,
 				Decoder: decoder,
 				Value:   fv,
 				Field:   ft,
@@ -124,7 +125,6 @@ func buildNode(v reflect.Value, cmd bool) *Node {
 				node.Flags = append(node.Flags, &Flag{
 					Value:       value,
 					Short:       short,
-					Default:     dflt,
 					Placeholder: placeholder,
 					Env:         env,
 				})
@@ -135,12 +135,8 @@ func buildNode(v reflect.Value, cmd bool) *Node {
 	// Scan through argument positionals to ensure optional is never before a required
 	last := true
 	for _, p := range node.Positional {
-		if p.Flag {
-			continue
-		}
-
 		if !last && p.Required {
-			fail("arguments can not be required after an optional: %v", p.Name)
+			fail("argument %q can not be required after an optional", p.Name)
 		}
 
 		last = p.Required

kong.go

@@ -75,10 +75,15 @@ func (k *Kong) reset(node *Node) {
 		flag.Value.Value.Set(reflect.Zero(flag.Value.Value.Type()))
 		if flag.Default != "" {
 			flag.Decode(Scan(flag.Default))
+			flag.Set = false
 		}
 	}
 	for _, pos := range node.Positional {
 		pos.Value.Set(reflect.Zero(pos.Value.Type()))
+		if pos.Default != "" {
+			pos.Decode(Scan(pos.Default))
+			pos.Set = false
+		}
 	}
 	for _, branch := range node.Children {
 		if branch.Argument != nil {
@@ -210,14 +215,14 @@ func (k *Kong) applyNode(scan *Scanner, node *Node, flags []*Flag) (command []st
 		return nil, err
 	}
 
-	if err := chickMissingFlags(node.Children, flags); err != nil {
+	if err := checkMissingFlags(node.Children, flags); err != nil {
 		return nil, err
 	}
 
 	return
 }
 
-func chickMissingFlags(children []*Branch, flags []*Flag) error {
+func checkMissingFlags(children []*Branch, flags []*Flag) error {
 	// Only check required missing fields at the last child.
 	if len(children) > 0 {
 		return nil
@@ -290,7 +295,6 @@ func matchFlags(flags []*Flag, token Token, scan *Scanner, matcher func(f *Flag)
 			if err != nil {
 				return err
 			}
-			flag.Set = true
 			return nil
 		}
 	}

kong_test.go

@@ -241,3 +241,13 @@ func TestMixedRequiredArgs(t *testing.T) {
 		require.Equal(t, "gak", cli.Name)
 	})
 }
+
+func TestDefaultValueForOptionalArg(t *testing.T) {
+	var cli struct {
+		Arg string `arg:"" optional:"" default:"default"`
+	}
+	p := mustNew(t, &cli)
+	_, err := p.Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, "default", cli.Arg)
+}

model.go

@@ -25,6 +25,7 @@ type Value struct {
 	Flag     bool // True if flag, false if positional argument.
 	Name     string
 	Help     string
+	Default  string
 	Decoder  Decoder
 	Field    reflect.StructField
 	Value    reflect.Value
@@ -34,7 +35,11 @@ type Value struct {
 }
 
 func (v *Value) Decode(scan *Scanner) error {
-	return v.Decoder.Decode(&DecoderContext{Value: v}, scan, v.Value)
+	err := v.Decoder.Decode(&DecoderContext{Value: v}, scan, v.Value)
+	if err == nil {
+		v.Set = true
+	}
+	return err
 }
 
 type Positional = Value
@@ -49,5 +54,4 @@ type Flag struct {
 	Placeholder string
 	Env         string
 	Short       rune
-	Default     string
 }