Add support for custom unmarshaling of strings

fig.go

@@ -28,6 +28,42 @@ const (
 	DefaultTimeLayout = time.RFC3339
 )
 
+// StringUnmarshaler is an interface for custom unmarshaling of strings
+//
+// If a field with a local type asignment satisfies this interface, it allows the user
+// to implment their own custom type unmarshaling method.
+//
+// Example:
+//
+//	type ListenerType uint
+//
+//	const (
+//	  ListenerUnix ListenerType = iota
+//	  ListenerTCP
+//	  ListenerTLS
+//	)
+//
+//	type Config struct {
+//	  Listener ListenerType `fig:"listener_type" default:"unix"`
+//	}
+//
+//	func (l *ListenerType) UnmarshalType(v string) error {
+//		   switch strings.ToLower(v) {
+//		   case "unix":
+//		     *l = ListenerUnix
+//		   case "tcp":
+//		     *l = ListenerTCP
+//		   case "tls":
+//		     *l = ListenerTLS
+//		   default:
+//	      return fmt.Errorf("unknown listener type: %s", v)
+//	    }
+//	    return nil
+//	 }
+type StringUnmarshaler interface {
+	UnmarshalString(s string) error
+}
+
 // Load reads a configuration file and loads it into the given struct. The
 // parameter `cfg` must be a pointer to a struct.
 //
@@ -158,6 +194,7 @@ func (f *fig) decodeMap(m map[string]interface{}, result interface{}) error {
 			mapstructure.StringToTimeDurationHookFunc(),
 			mapstructure.StringToTimeHookFunc(f.timeLayout),
 			stringToRegexpHookFunc(),
+			stringToStringUnmarshalerHook(),
 		),
 	})
 	if err != nil {
@@ -183,6 +220,36 @@ func stringToRegexpHookFunc() mapstructure.DecodeHookFunc {
 	}
 }
 
+// stringToStringUnmarshalerHook returns a DecodeHookFunc that executes a custom method which
+// satisfies the StringUnmarshaler interface on custom types.
+func stringToStringUnmarshalerHook() mapstructure.DecodeHookFunc {
+	return func(f reflect.Type, t reflect.Type, data interface{}) (interface{}, error) {
+		if f.Kind() != reflect.String {
+			return data, nil
+		}
+
+		ds, ok := data.(string)
+		if !ok {
+			return data, nil
+		}
+
+		if reflect.PointerTo(t).Implements(reflect.TypeOf((*StringUnmarshaler)(nil)).Elem()) {
+			val := reflect.New(t).Interface()
+
+			if unmarshaler, ok := val.(StringUnmarshaler); ok {
+				err := unmarshaler.UnmarshalString(ds)
+				if err != nil {
+					return nil, err
+				}
+
+				return reflect.ValueOf(val).Elem().Interface(), nil
+			}
+		}
+
+		return data, nil
+	}
+}
+
 // processCfg processes a cfg struct after it has been loaded from
 // the config file, by validating required fields and setting defaults
 // where applicable.

fig_test.go

@@ -84,6 +84,14 @@ type Item struct {
 	Path string `fig:"path" validate:"required"`
 }
 
+type ListenerType uint
+
+const (
+	ListenerUnix ListenerType = iota
+	ListenerTCP
+	ListenerTLS
+)
+
 func validPodConfig() Pod {
 	var pod Pod
 
@@ -249,6 +257,9 @@ func Test_fig_Load_Defaults(t *testing.T) {
 					Application struct {
 						BuildDate time.Time `fig:"build_date" default:"2020-01-01T12:00:00Z"`
 					}
+					Server struct {
+						Listener ListenerType `fig:"listener_type" default:"unix"`
+					}
 				}
 
 				var want Server
@@ -259,6 +270,7 @@ func Test_fig_Load_Defaults(t *testing.T) {
 				want.Logger.Production = false
 				want.Logger.Metadata.Keys = []string{"ts"}
 				want.Application.BuildDate = time.Date(2020, 1, 1, 12, 0, 0, 0, time.UTC)
+				want.Server.Listener = ListenerTCP
 
 				var cfg Server
 				err := Load(&cfg, File(f), Dirs(filepath.Join("testdata", "valid")))
@@ -590,17 +602,19 @@ func Test_fig_decodeMap(t *testing.T) {
 		"log_level": "debug",
 		"severity":  "5",
 		"server": map[string]interface{}{
-			"ports":  []int{443, 80},
-			"secure": 1,
+			"ports":         []int{443, 80},
+			"secure":        1,
+			"listener_type": "tls",
 		},
 	}
 
 	var cfg struct {
 		Level    string `fig:"log_level"`
 		Severity int    `fig:"severity" validate:"required"`
 		Server   struct {
-			Ports  []string `fig:"ports" default:"[443]"`
-			Secure bool
+			Ports    []string `fig:"ports" default:"[443]"`
+			Secure   bool
+			Listener ListenerType `fig:"listener_type" default:"unix"`
 		} `fig:"server"`
 	}
 
@@ -623,6 +637,10 @@ func Test_fig_decodeMap(t *testing.T) {
 	if cfg.Server.Secure == false {
 		t.Error("cfg.Server.Secure == false")
 	}
+
+	if cfg.Server.Listener != ListenerTLS {
+		t.Errorf("cfg.Server.Listener: want: %s, got: %s", ListenerTLS, cfg.Server.Listener)
+	}
 }
 
 func Test_fig_processCfg(t *testing.T) {
@@ -1263,3 +1281,30 @@ func setenv(t *testing.T, key, value string) {
 	t.Helper()
 	t.Setenv(key, value)
 }
+
+func (l *ListenerType) UnmarshalString(v string) error {
+	switch strings.ToLower(v) {
+	case "unix":
+		*l = ListenerUnix
+	case "tcp":
+		*l = ListenerTCP
+	case "tls":
+		*l = ListenerTLS
+	default:
+		return fmt.Errorf("unknown listener type: %s", v)
+	}
+	return nil
+}
+
+func (l ListenerType) String() string {
+	switch l {
+	case ListenerUnix:
+		return "unix"
+	case ListenerTCP:
+		return "tcp"
+	case ListenerTLS:
+		return "tls"
+	default:
+		return "unknown"
+	}
+}