Add NotFound()

README.md

@@ -307,7 +307,10 @@ type delegatingMappers struct {
 
 func (d *delegatingMappers) Get(name string) (any, error) {
 	v, err := d.Mappers.Get(name)
-	if err != nil && strings.Contains(err.Error(), "not found") {
+	var merr interface {
+		NotFound() bool
+	}
+	if errors.As(err, &merr) && merr.NotFound() {
 		return d.parent.Get(name)
 	}
 	return v, err

mappers.go

@@ -43,6 +43,19 @@ func (s *singleton[T]) MustGet() T {
 	return instance
 }
 
+type merror struct {
+	error
+	notFound bool
+}
+
+func (e *merror) NotFound() bool {
+	return e.notFound
+}
+
+func (e *merror) Unwrap() error {
+	return e.error
+}
+
 type MapperGetter = ` + mapperGetterSrc + `
 
 // Mappers is a collection of mappers.
@@ -141,11 +154,19 @@ func (d *mappers) Get(name string) (any, error) {
 		if fok && factory != nil {
 			obj, err := factory.(func(*mappers) (any, error))(d)
 			if err != nil {
-				return nil, fmt.Errorf("Failed to create a mapper: %%w", err)
+				merr := &merror {
+					error: err,
+					notFound: false,
+				}
+				return nil, fmt.Errorf("Failed to create a mapper: %%w", merr)
 			}
 			d.dependencies.Store(name, obj)
 		} else {
-			return nil, fmt.Errorf("Object %%s not found", name)
+			merr := &merror {
+				error: fmt.Errorf("Object %%s not found", name),
+				notFound: true,
+			}
+			return nil, merr
 		}
 	}
 	obj, _ := d.dependencies.Load(name)

testdata/testmod/mapper/todo_mapper_test.go

@@ -1,6 +1,7 @@
 package mapper_test
 
 import (
+	"errors"
 	"testing"
 	"time"
 
@@ -241,3 +242,14 @@ func TestNilCollection(t *testing.T) {
 	}
 
 }
+
+func TestMapperNotFound(t *testing.T) {
+	mappers := NewMappers()
+	_, err := mappers.Get("Dummy")
+	var merr interface {
+		NotFound() bool
+	}
+	if !errors.As(err, &merr) || !merr.NotFound() {
+		t.Errorf("error should be a not found error")
+	}
+}