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

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()})
+	}
+}