errors.Error is now an interface, implementation is hidden

errors/errors.go

@@ -87,29 +87,44 @@ func (pos Position) Union (other Position) Position {
 }
 
 // Error is an error that contains positional information.
-type Error struct {
-	Position
-	Message string
+type Error interface {
+	error
+	Position () Position
 }
-var _ error = new(Error)
 
-func (err *Error) Error () string {
+type errInternal struct {
+	position Position
+	message  string
+}
+var _ Error = new(errInternal)
+
+func (err *errInternal) Error () string {
 	return err.String()
 }
 
-func (err *Error) String () string {
-	return fmt.Sprintf("%v: %s", err.Position, err.Message)
+func (err *errInternal) String () string {
+	return fmt.Sprintf("%v: %s", err.position, err.message)
 }
 
-// Format produces a formatted printout of the error, and its location.
-func (err *Error) Format () string {
-	return fmt.Sprintf("%v\n%v", err, err.Position.Format())
+func (err *errInternal) Position () Position {
+	return err.position
 }
 
 // Errorf produces an error with a fmt'd message.
-func Errorf (position Position, format string, variables ...any) error {
-	return &Error {
-		Position: position,
-		Message:  fmt.Sprintf(format, variables...),
+func Errorf (position Position, format string, variables ...any) Error {
+	return &errInternal {
+		position: position,
+		message:  fmt.Sprintf(format, variables...),
+	}
+}
+
+// Format returns a formatted string representing an error. This string may take
+// up multiple lines and contain ANSI escape codes. If err does not fulfill the
+// Error interface, err.Error() is returned instead.
+func Format (err error) string {
+	if err, ok := err.(Error); ok {
+		return fmt.Sprintf("%v\n%v", err.Error(), err.Position().Format())
+	} else {
+		return err.Error()
 	}
 }

errors/errors_test.go

@@ -8,16 +8,15 @@ testError (test,
 `example.fspl:11:7: some error
 11   | lorem ipsum dolor
              ^^^^^`,
-&Error {
-	Position: Position {
+Errorf (
+	Position {
 		File: "example.fspl",
 		Line: "lorem ipsum dolor",
 		Row:   10,
 		Start:  6,
 		End:   11,
 	},
-	Message: "some error",
-})
+	"some error"))
 }
 
 func TestErrorTab (test *testing.T) {
@@ -25,16 +24,15 @@ testError (test,
 `example.fspl:11:8: some error
 11   |         lorem   ipsum   dolor
                        ^^^^^`,
-&Error {
-	Position: Position {
+Errorf (
+	Position {
 		File: "example.fspl",
 		Line: "\tlorem\tipsum\tdolor",
 		Row:   10,
 		Start:  7,
 		End:   12,
 	},
-	Message: "some error",
-})
+	"some error"))
 }
 
 func TestErrorTabInBetween (test *testing.T) {
@@ -42,16 +40,15 @@ testError (test,
 `example.fspl:11:8: some error
 11   |         lorem   ipsum   dolor
                        ^^^^^^^^^`,
-&Error {
-	Position: Position {
+Errorf (
+	Position {
 		File: "example.fspl",
 		Line: "\tlorem\tipsum\tdolor",
 		Row:   10,
 		Start:  7,
 		End:   14,
 	},
-	Message: "some error",
-})
+	"some error"))
 }
 
 func TestGetXInTabbedString (test *testing.T) {

errors/test-common.go

@@ -11,6 +11,6 @@ func testString (test *testing.T, correct string, got string) {
 	testcommon.Compare(test, correct, got)
 }
 
-func testError (test *testing.T, correct string, err *Error) {
-	testString(test, correct, err.Format())
+func testError (test *testing.T, correct string, err Error) {
+	testString(test, correct, Format(err))
 }