add supressed loader initialization function

Currently there is no way to use the suppressed loader, as the group attribute is always nil (issue #86). This adds a new function which initializes the structure with the Loader and group parameters, if the group parameter is nil, it creates a new single flight group object.

Author: davseby

cache.go

@@ -538,9 +538,22 @@ func (l LoaderFunc[K, V]) Load(c *Cache[K, V], key K) *Item[K, V] {
 // SuppressedLoader wraps another Loader and suppresses duplicate
 // calls to its Load method.
 type SuppressedLoader[K comparable, V any] struct {
-	Loader[K, V]
+	loader Loader[K, V]
+	group  *singleflight.Group
+}
+
+// NewSuppressedLoader creates a new instance of suppressed loader.
+// If the group parameter is nil, a newly created instance of
+// *singleflight.Group is used.
+func NewSuppressedLoader[K comparable, V any](loader Loader[K, V], group *singleflight.Group) *SuppressedLoader[K, V] {
+	if group == nil {
+		group = &singleflight.Group{}
+	}
 
-	group *singleflight.Group
+	return &SuppressedLoader[K, V]{
+		loader: loader,
+		group:  group,
+	}
 }
 
 // Load executes a custom item retrieval logic and returns the item that
@@ -559,7 +572,7 @@ func (l *SuppressedLoader[K, V]) Load(c *Cache[K, V], key K) *Item[K, V] {
 	// the error that we return ourselves in the func below, which
 	// is also nil
 	res, _, _ := l.group.Do(strKey, func() (interface{}, error) {
-		item := l.Loader.Load(c, key)
+		item := l.loader.Load(c, key)
 		if item == nil {
 			return nil, nil
 		}

cache_test.go

@@ -945,6 +945,40 @@ func Test_LoaderFunc_Load(t *testing.T) {
 	assert.True(t, called)
 }
 
+func Test_NewSuppressedLoader(t *testing.T) {
+	var called bool
+
+	loader := LoaderFunc[string, string](func(_ *Cache[string, string], _ string) *Item[string, string] {
+		called = true
+		return nil
+	})
+
+	// uses the provided loader and group parameters
+	group := &singleflight.Group{}
+
+	sl := NewSuppressedLoader[string, string](loader, group)
+	require.NotNil(t, sl)
+	require.NotNil(t, sl.loader)
+
+	sl.loader.Load(nil, "")
+
+	assert.True(t, called)
+	assert.Equal(t, group, sl.group)
+
+	// uses the provided loader and automatically creates a new instance
+	// of *singleflight.Group as nil parameter is passed
+	called = false
+
+	sl = NewSuppressedLoader[string, string](loader, nil)
+	require.NotNil(t, sl)
+	require.NotNil(t, sl.loader)
+
+	sl.loader.Load(nil, "")
+
+	assert.True(t, called)
+	assert.NotNil(t, group, sl.group)
+}
+
 func Test_SuppressedLoader_Load(t *testing.T) {
 	var (
 		mu        sync.Mutex
@@ -954,7 +988,7 @@ func Test_SuppressedLoader_Load(t *testing.T) {
 	)
 
 	l := SuppressedLoader[string, string]{
-		Loader: LoaderFunc[string, string](func(_ *Cache[string, string], _ string) *Item[string, string] {
+		loader: LoaderFunc[string, string](func(_ *Cache[string, string], _ string) *Item[string, string] {
 			mu.Lock()
 			loadCalls++
 			mu.Unlock()