Once with ret based on golang/go#56102

sync.go

@@ -0,0 +1,14 @@
+package genh
+
+import "sync"
+
+type Once[T any] struct {
+	once sync.Once
+	v    T
+	err  error
+}
+
+func (o *Once[T]) Do(fn func() (T, error)) (v T, err error) {
+	o.once.Do(func() { o.v, o.err = fn() })
+	return o.v, o.err
+}