From 794a835e83eef06d3a43999c4e33fc01ce3b5a2e Mon Sep 17 00:00:00 2001
From: Denise Li <dli@squareup.com>
Date: Thu, 18 Apr 2024 13:20:43 -0400
Subject: [PATCH] feat: Create Singleton func for convenient singleton
 construction (#1297)

Fixes issue https://github.com/TBD54566975/ftl/issues/1295

---------

Co-authored-by: Alec Thomas <aat@tbd.email>
---
 go-runtime/ftl/singleton.go      | 27 ++++++++++++++++++++++++++
 go-runtime/ftl/singleton_test.go | 33 ++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 go-runtime/ftl/singleton.go
 create mode 100644 go-runtime/ftl/singleton_test.go

diff --git a/go-runtime/ftl/singleton.go b/go-runtime/ftl/singleton.go
new file mode 100644
index 000000000..ffffbe793
--- /dev/null
+++ b/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{}}
+}
diff --git a/go-runtime/ftl/singleton_test.go b/go-runtime/ftl/singleton_test.go
new file mode 100644
index 000000000..996eb0d47
--- /dev/null
+++ b/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)
+	})
+}