Allow to use values (not pointers) with TextUnmarshaler

The patch makes sure that both values and pointer to values are checked
for custom TextUnmarshal implementation. This will allow to use go-arg
custom parsing as follows:

var args struct {
  Arg CustomType
}

instead of

var args struct {
  Arg *CustomType
}

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

scalar.go

@@ -47,6 +47,13 @@ func ParseValue(v reflect.Value, s string) error {
 	if scalar, ok := v.Interface().(encoding.TextUnmarshaler); ok {
 		return scalar.UnmarshalText([]byte(s))
 	}
+	// If it's a value instead of a pointer, check that we can unmarshal it
+	// via TextUnmarshaler as well
+	if v.CanAddr() {
+		if scalar, ok := v.Addr().Interface().(encoding.TextUnmarshaler); ok {
+			return scalar.UnmarshalText([]byte(s))
+		}
+	}
 
 	// If we have a pointer then dereference it
 	if v.Kind() == reflect.Ptr {
@@ -126,7 +133,7 @@ func ParseValue(v reflect.Value, s string) error {
 // CanParse returns true if the type can be parsed from a string.
 func CanParse(t reflect.Type) bool {
 	// If it implements encoding.TextUnmarshaler then use that
-	if t.Implements(textUnmarshalerType) {
+	if t.Implements(textUnmarshalerType) || reflect.PtrTo(t).Implements(textUnmarshalerType) {
 		return true
 	}
 

scalar_test.go

@@ -10,6 +10,15 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+type textUnmarshaler struct {
+	val int
+}
+
+func (f *textUnmarshaler) UnmarshalText(b []byte) error {
+	f.val = len(b)
+	return nil
+}
+
 func assertParse(t *testing.T, expected interface{}, str string) {
 	v := reflect.New(reflect.TypeOf(expected)).Elem()
 	err := ParseValue(v, str)
@@ -67,6 +76,9 @@ func TestParseValue(t *testing.T) {
 
 	// MAC addresses
 	assertParse(t, net.HardwareAddr("\x01\x23\x45\x67\x89\xab"), "01:23:45:67:89:ab")
+
+	// custom text unmarshaler
+	assertParse(t, textUnmarshaler{3}, "abc")
 }
 
 func TestParse(t *testing.T) {