Add a kong.ApplyDefaults() function.

defaults.go

@@ -0,0 +1,11 @@
+package kong
+
+// ApplyDefaults applies defaults to a struct.
+func ApplyDefaults(target interface{}, options ...Option) error {
+	app, err := New(target, options...)
+	if err != nil {
+		return err
+	}
+	_, err = app.Parse(nil)
+	return err
+}

defaults_test.go

@@ -0,0 +1,19 @@
+package kong
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestApplyDefaults(t *testing.T) {
+	type CLI struct {
+		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)
+}