feat: Create Singleton func for convenient singleton construction (#1297

)

Fixes issue #1295

---------

Co-authored-by: Alec Thomas <[email protected]>

go-runtime/ftl/singleton.go

@@ -0,0 +1,27 @@
+package ftl
+
+import (
+	"context"
+	"sync"
+)
+
+type SingletonHandle[T any] struct {
+	fn   func(context.Context) (T, error)
+	out  T
+	once *sync.Once
+}
+
+func (sh *SingletonHandle[T]) Get(ctx context.Context) T {
+	sh.once.Do(func() {
+		t, err := sh.fn(ctx)
+		if err != nil {
+			panic(err)
+		}
+		sh.out = t
+	})
+	return sh.out
+}
+
+func Singleton[T any](fn func(context.Context) (T, error)) SingletonHandle[T] {
+	return SingletonHandle[T]{fn: fn, once: &sync.Once{}}
+}

go-runtime/ftl/singleton_test.go

@@ -0,0 +1,33 @@
+package ftl
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	"github.com/alecthomas/assert/v2"
+)
+
+func TestSingletonBaseCase(t *testing.T) {
+	incrementer := 0
+
+	ctx := context.Background()
+	once := Singleton[string](func(ctx context.Context) (string, error) {
+		incrementer++
+		return "only once", nil
+	})
+
+	assert.Equal(t, once.Get(ctx), "only once")
+	assert.Equal(t, once.Get(ctx), "only once")
+	assert.Equal(t, incrementer, 1)
+}
+
+func TestSingletonPanic(t *testing.T) {
+	ctx := context.Background()
+	once := Singleton[string](func(ctx context.Context) (string, error) {
+		return "", fmt.Errorf("test error")
+	})
+	assert.Panics(t, func() {
+		once.Get(ctx)
+	})
+}