Proposal: expose MultiError (#8)

One of the great things about semgroup over x/sync/errgroup is that it
actually waits all tasks to complete and returns an error.

However, the returned error type doesn't let the caller to iterate over the
errors (e.g. my use case requires me to truncate the list of errors).

Therefore, introducing a backwards-compatible change that exports `MultiError`
type, and guarantees that `Wait()` method returns an error of this type.

semgroup.go

@@ -25,7 +25,7 @@ type Group struct {
 	wg  sync.WaitGroup
 	ctx context.Context
 
-	errs multiError
+	errs MultiError
 	mu   sync.Mutex // protects errs
 }
 
@@ -70,15 +70,17 @@ func (g *Group) Go(f func() error) {
 }
 
 // Wait blocks until all function calls from the Go method have returned, then
-// returns all accumulated non-nil error (if any) from them.
+// returns all accumulated non-nil errors (if any) from them.
+//
+// If a non-nil error is returned, it will be of type [MultiError].
 func (g *Group) Wait() error {
 	g.wg.Wait()
 	return g.errs.ErrorOrNil()
 }
 
-type multiError []error
+type MultiError []error
 
-func (e multiError) Error() string {
+func (e MultiError) Error() string {
 	var b strings.Builder
 	fmt.Fprintf(&b, "%d error(s) occurred:\n", len(e))
 
@@ -92,15 +94,16 @@ func (e multiError) Error() string {
 	return b.String()
 }
 
-func (e multiError) ErrorOrNil() error {
+// ErrorOrNil returns nil if there are no errors, otherwise returns itself.
+func (e MultiError) ErrorOrNil() error {
 	if len(e) == 0 {
 		return nil
 	}
 
 	return e
 }
 
-func (e multiError) Is(target error) bool {
+func (e MultiError) Is(target error) bool {
 	for _, err := range e {
 		if errors.Is(err, target) {
 			return true
@@ -109,7 +112,7 @@ func (e multiError) Is(target error) bool {
 	return false
 }
 
-func (e multiError) As(target interface{}) bool {
+func (e MultiError) As(target interface{}) bool {
 	for _, err := range e {
 		if errors.As(err, target) {
 			return true

semgroup_test.go

@@ -62,6 +62,9 @@ func TestGroup_multiple_tasks_errors(t *testing.T) {
 	if err == nil {
 		t.Fatalf("g.Wait() should return an error")
 	}
+	if !errors.As(err, &MultiError{}) {
+		t.Fatalf("the error should be of type MultiError")
+	}
 
 	wantErr := `2 error(s) occurred:
 * foo
@@ -124,6 +127,15 @@ func TestGroup_multiple_tasks_errors_Is(t *testing.T) {
 	if errors.Is(err, bazErr) {
 		t.Errorf("error should not be contained %v\n", bazErr)
 	}
+
+	var gotMultiErr MultiError
+	if !errors.As(err, &gotMultiErr) {
+		t.Fatalf("error should be matched MultiError")
+	}
+	expectedErr := (MultiError{fooErr, barErr}).Error()
+	if gotMultiErr.Error() != expectedErr {
+		t.Errorf("error should be %q, got %q", expectedErr, gotMultiErr.Error())
+	}
 }
 
 type foobarErr struct{ str string }