Remove the DataErrorf constructor.

Make Errorf return the *jrpc2.Error value concretely, and equip that type with
a WithData method. This permits the reuse of base errors with different data.

base.go

@@ -360,7 +360,7 @@ func (j *jmessage) parseJSON(data []byte) error {
 
 	// Report an error for extraneous fields.
 	if j.err == nil && len(extra) != 0 {
-		j.err = DataErrorf(code.InvalidRequest, extra, "extra fields in request")
+		j.err = Errorf(code.InvalidRequest, "extra fields in request").WithData(extra)
 	}
 	return nil
 }

error.go

@@ -26,6 +26,18 @@ func (e Error) MarshalJSON() ([]byte, error) {
 	return json.Marshal(jerror{C: int32(e.Code), M: e.Message, D: e.Data})
 }
 
+// WithData marshals v as JSON and constructs a copy of e whose Data field
+// includes the result. If v == nil or if marshaling v fails, e is returned
+// without modification.
+func (e *Error) WithData(v interface{}) *Error {
+	if v == nil {
+		return e
+	} else if data, err := json.Marshal(v); err == nil {
+		return &Error{Code: e.Code, Message: e.Message, Data: data}
+	}
+	return e
+}
+
 // UnmarshalJSON implements the json.Unmarshaler interface for Error values.
 func (e *Error) UnmarshalJSON(data []byte) error {
 	var v jerror
@@ -62,20 +74,6 @@ var ErrConnClosed = errors.New("client connection is closed")
 
 // Errorf returns an error value of concrete type *Error having the specified
 // code and formatted message string.
-// It is shorthand for DataErrorf(code, nil, msg, args...)
-func Errorf(code code.Code, msg string, args ...interface{}) error {
-	return DataErrorf(code, nil, msg, args...)
-}
-
-// DataErrorf returns an error value of concrete type *Error having the
-// specified code, error data, and formatted message string.
-// If v == nil this behaves identically to Errorf(code, msg, args...).
-func DataErrorf(code code.Code, v interface{}, msg string, args ...interface{}) error {
-	e := &Error{Code: code, Message: fmt.Sprintf(msg, args...)}
-	if v != nil {
-		if data, err := json.Marshal(v); err == nil {
-			e.Data = data
-		}
-	}
-	return e
+func Errorf(code code.Code, msg string, args ...interface{}) *Error {
+	return &Error{Code: code, Message: fmt.Sprintf(msg, args...)}
 }

internal_test.go

@@ -292,7 +292,7 @@ func TestMarshalResponse(t *testing.T) {
 	}{
 		{"", nil, "", `{"jsonrpc":"2.0"}`},
 		{"null", nil, "", `{"jsonrpc":"2.0","id":null}`},
-		{"123", Errorf(code.ParseError, "failed").(*Error), "",
+		{"123", Errorf(code.ParseError, "failed"), "",
 			`{"jsonrpc":"2.0","id":123,"error":{"code":-32700,"message":"failed"}}`},
 		{"456", nil, `{"ok":true,"values":[4,5,6]}`,
 			`{"jsonrpc":"2.0","id":456,"result":{"ok":true,"values":[4,5,6]}}`},

jrpc2_test.go

@@ -435,7 +435,7 @@ func TestError_withData(t *testing.T) {
 	const errMessage = "error thingy"
 	loc := server.NewLocal(handler.Map{
 		"Err": handler.New(func(_ context.Context) (int, error) {
-			return 17, jrpc2.DataErrorf(errCode, json.RawMessage(errData), errMessage)
+			return 17, jrpc2.Errorf(errCode, errMessage).WithData(json.RawMessage(errData))
 		}),
 		"Push": handler.New(func(ctx context.Context) (bool, error) {
 			return false, jrpc2.ServerFromContext(ctx).Notify(ctx, "PushBack", nil)