Merge pull request #67 from cdr/ammar

Add reflection encoder for lists

Author: nhooyr

map.go

@@ -13,13 +13,29 @@ import (
 // Map represents an ordered map of fields.
 type Map []Field
 
+// SlogValue implements Value.
+func (m Map) SlogValue() interface{} {
+	return ForceJSON(m)
+}
+
 var _ json.Marshaler = Map(nil)
 
 // MarshalJSON implements json.Marshaler.
 //
 // It is guaranteed to return a nil error.
-// Any error marshalling a field will
-// become the field's value.
+// Any error marshalling a field will become the field's value.
+//
+// Every field value is encoded with the following process:
+//
+// 1. slog.Value is handled to allow any type to replace its representation for logging.
+//
+// 2. xerrors.Formatter is handled.
+//
+// 3. error and fmt.Stringer are handled.
+//
+// 4. slices and arrays are handled to go through the encode function for every value.
+//
+// 5. json.Marshal is invoked as the default case.
 func (m Map) MarshalJSON() ([]byte, error) {
 	b := &bytes.Buffer{}
 	b.WriteByte('{')
@@ -55,14 +71,14 @@ func (v jsonVal) MarshalJSON() ([]byte, error) {
 	return json.Marshal(v.v)
 }
 
-func marshalArray(a []interface{}) []byte {
+func marshalList(rv reflect.Value) []byte {
 	b := &bytes.Buffer{}
 	b.WriteByte('[')
-	for i, v := range a {
+	for i := 0; i < rv.Len(); i++ {
 		b.WriteByte('\n')
-		b.Write(encode(v))
+		b.Write(encode(rv.Index(i).Interface()))
 
-		if i < len(a)-1 {
+		if i < rv.Len()-1 {
 			b.WriteByte(',')
 		}
 	}
@@ -75,13 +91,24 @@ func encode(v interface{}) []byte {
 	switch v := v.(type) {
 	case Value:
 		return encode(v.SlogValue())
-	case []interface{}:
-		return marshalArray(v)
 	case xerrors.Formatter:
 		return encode(errorChain(v))
 	case error, fmt.Stringer:
 		return encode(fmt.Sprint(v))
 	default:
+		rv := reflect.Indirect(reflect.ValueOf(v))
+		if rv.IsValid() {
+			switch rv.Type().Kind() {
+			case reflect.Slice:
+				if rv.IsNil() {
+					break
+				}
+				fallthrough
+			case reflect.Array:
+				return marshalList(rv)
+			}
+		}
+
 		b, err := json.Marshal(v)
 		if err != nil {
 			return encode(M(

map_test.go

@@ -93,7 +93,7 @@ func TestMap(t *testing.T) {
 					{
 						"msg": "failed to marshal to JSON",
 						"fun": "cdr.dev/slog.encode",
-						"loc": "`+mapTestFile+`:88"
+						"loc": "`+mapTestFile+`:115"
 					},
 					"json: unsupported type: func(*testing.T, string) string"
 				],
@@ -164,6 +164,16 @@ func TestMap(t *testing.T) {
 			"error": "xdxd"
 		}`)
 	})
+
+	t.Run("nilSlice", func(t *testing.T) {
+		t.Parallel()
+
+		test(t, slog.M(
+			slog.F("slice", []string(nil)),
+		), `{
+			"slice": null
+		}`)
+	})
 }
 
 type meow struct {