Document encoding

Author: nhooyr

map.go

@@ -23,8 +23,19 @@ 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('{')
@@ -90,8 +101,7 @@ func encode(v interface{}) []byte {
 			switch rv.Type().Kind() {
 			case reflect.Slice:
 				if rv.IsNil() {
-					b, _ := json.Marshal(nil)
-					return b
+					break
 				}
 				fallthrough
 			case reflect.Array:

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+`:105"
+						"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 {