ApplyDefaults() now only applies defaults if the value is not already otherwise set.

context.go

@@ -60,8 +60,8 @@ type Context struct {
 //
 // The returned Context will include a Path of all commands, arguments, positionals and flags.
 //
-// This just constructs a new trace. To fully apply the trace you must call Resolve(), Validate() and
-// Apply().
+// This just constructs a new trace. To fully apply the trace you must call Reset(), Resolve(),
+// Validate() and Apply().
 func Trace(k *Kong, args []string) (*Context, error) {
 	c := &Context{
 		Kong: k,
@@ -74,11 +74,6 @@ func Trace(k *Kong, args []string) (*Context, error) {
 		bindings: bindings{},
 	}
 	c.Error = c.trace(c.Model.Node)
-	err := c.reset(c.Model.Node)
-	if err != nil {
-		return nil, err
-	}
-
 	return c, nil
 }
 
@@ -236,9 +231,9 @@ func (c *Context) FlagValue(flag *Flag) interface{} {
 	return flag.DefaultValue.Interface()
 }
 
-// Recursively reset values to defaults (as specified in the grammar) or the zero value.
-func (c *Context) reset(node *Node) error {
-	return Visit(node, func(node Visitable, next Next) error {
+// Reset recursively resets values to defaults (as specified in the grammar) or the zero value.
+func (c *Context) Reset() error {
+	return Visit(c.Model.Node, func(node Visitable, next Next) error {
 		if value, ok := node.(*Value); ok {
 			return next(value.Reset())
 		}
@@ -441,6 +436,28 @@ func (c *Context) getValue(value *Value) reflect.Value {
 	return v
 }
 
+// ApplyDefaults if they are not already set.
+func (c *Context) ApplyDefaults() error {
+	return Visit(c.Model.Node, func(node Visitable, next Next) error {
+		var value *Value
+		switch node := node.(type) {
+		case *Flag:
+			value = node.Value
+		case *Node:
+			value = node.Argument
+		case *Value:
+			value = node
+		default:
+		}
+		if value != nil {
+			if err := value.ApplyDefault(); err != nil {
+				return err
+			}
+		}
+		return next(nil)
+	})
+}
+
 // Apply traced context to the target grammar.
 func (c *Context) Apply() (string, error) {
 	path := []string{}

defaults.go

@@ -1,11 +1,22 @@
 package kong
 
-// ApplyDefaults applies defaults to a struct.
+// ApplyDefaults if they are not already set.
 func ApplyDefaults(target interface{}, options ...Option) error {
 	app, err := New(target, options...)
 	if err != nil {
 		return err
 	}
-	_, err = app.Parse(nil)
-	return err
+	ctx, err := Trace(app, nil)
+	if err != nil {
+		return err
+	}
+	err = ctx.Resolve()
+	if err != nil {
+		return err
+	}
+	err = ctx.Validate()
+	if err != nil {
+		return err
+	}
+	return ctx.ApplyDefaults()
 }

defaults_test.go

@@ -12,8 +12,23 @@ func TestApplyDefaults(t *testing.T) {
 		Str      string        `default:"str"`
 		Duration time.Duration `default:"30s"`
 	}
-	cli := &CLI{}
-	err := ApplyDefaults(cli)
-	require.NoError(t, err)
-	require.Equal(t, &CLI{Str: "str", Duration: time.Second * 30}, cli)
+	tests := []struct {
+		name     string
+		target   CLI
+		expected CLI
+	}{
+		{name: "DefaultsWhenNotSet",
+			expected: CLI{Str: "str", Duration: time.Second * 30}},
+		{name: "PartiallySetDefaults",
+			target:   CLI{Duration: time.Second},
+			expected: CLI{Str: "str", Duration: time.Second}},
+	}
+	for _, tt := range tests {
+		// nolint: scopelint
+		t.Run(tt.name, func(t *testing.T) {
+			err := ApplyDefaults(&tt.target)
+			require.NoError(t, err)
+			require.Equal(t, tt.expected, tt.target)
+		})
+	}
 }

kong.go

@@ -197,6 +197,9 @@ func (k *Kong) Parse(args []string) (ctx *Context, err error) {
 	if ctx.Error != nil {
 		return nil, &ParseError{error: ctx.Error, Context: ctx}
 	}
+	if err = ctx.Reset(); err != nil {
+		return nil, &ParseError{error: err, Context: ctx}
+	}
 	if err = k.applyHook(ctx, "BeforeResolve"); err != nil {
 		return nil, &ParseError{error: err, Context: ctx}
 	}

model.go

@@ -2,6 +2,7 @@ package kong
 
 import (
 	"fmt"
+	"math"
 	"os"
 	"reflect"
 	"strconv"
@@ -308,6 +309,15 @@ func (v *Value) Apply(value reflect.Value) {
 	v.Set = true
 }
 
+// ApplyDefault value to field if it is not already set.
+func (v *Value) ApplyDefault() error {
+	if reflectValueIsZero(v.Target) {
+		return v.Reset()
+	}
+	v.Set = true
+	return nil
+}
+
 // Reset this value to its default, either the zero value or the parsed result of its envar,
 // or its "default" tag.
 //
@@ -376,3 +386,42 @@ func (f *Flag) FormatPlaceHolder() string {
 	}
 	return strings.ToUpper(f.Name) + tail
 }
+
+// This is directly from the Go 1.13 source code.
+func reflectValueIsZero(v reflect.Value) bool {
+	switch v.Kind() {
+	case reflect.Bool:
+		return !v.Bool()
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return v.Int() == 0
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return v.Uint() == 0
+	case reflect.Float32, reflect.Float64:
+		return math.Float64bits(v.Float()) == 0
+	case reflect.Complex64, reflect.Complex128:
+		c := v.Complex()
+		return math.Float64bits(real(c)) == 0 && math.Float64bits(imag(c)) == 0
+	case reflect.Array:
+		for i := 0; i < v.Len(); i++ {
+			if !reflectValueIsZero(v.Index(i)) {
+				return false
+			}
+		}
+		return true
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice, reflect.UnsafePointer:
+		return v.IsNil()
+	case reflect.String:
+		return v.Len() == 0
+	case reflect.Struct:
+		for i := 0; i < v.NumField(); i++ {
+			if !reflectValueIsZero(v.Field(i)) {
+				return false
+			}
+		}
+		return true
+	default:
+		// This should never happens, but will act as a safeguard for
+		// later, as a default value doesn't makes sense here.
+		panic(&reflect.ValueError{"reflect.Value.IsZero", v.Kind()})
+	}
+}

visit.go

@@ -5,7 +5,9 @@ import (
 )
 
 // Next should be called by Visitor to proceed with the walk.
-type Next func(error) error
+//
+// The walk will terminate if "err" is non-nil.
+type Next func(err error) error
 
 // Visitor can be used to walk all nodes in the model.
 type Visitor func(node Visitable, next Next) error