A nicer implementation of sync.Once

Doesnt require storing the result of the function in a dedicated global
variable with a dedicated getter function

tools/utils/once.go

@@ -0,0 +1,35 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"fmt"
+	"sync"
+	"sync/atomic"
+)
+
+var _ = fmt.Print
+
+type Once[T any] struct {
+	done       uint32
+	mutex      sync.Mutex
+	cached_val T
+
+	Run func() T
+}
+
+func (self *Once[T]) Get() T {
+	if atomic.LoadUint32(&self.done) == 0 {
+		self.do_slow()
+	}
+	return self.cached_val
+}
+
+func (self *Once[T]) do_slow() {
+	self.mutex.Lock()
+	defer self.mutex.Unlock()
+	if atomic.LoadUint32(&self.done) == 0 {
+		defer atomic.StoreUint32(&self.done, 1)
+		self.cached_val = self.Run()
+	}
+}

tools/utils/once_test.go

@@ -0,0 +1,24 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"fmt"
+	"testing"
+)
+
+var _ = fmt.Print
+
+func TestOnce(t *testing.T) {
+	num := 0
+	var G = (&Once[string]{Run: func() string {
+		num++
+		return fmt.Sprintf("%d", num)
+	}}).Get
+	G()
+	G()
+	G()
+	if num != 1 {
+		t.Fatalf("num unexpectedly: %d", num)
+	}
+}