2016 Update

Task/Singleton/Go/singleton-1.go

@@ -1,9 +1,31 @@
-package singlep
+package main
 
-// package level data declarations serve as singleton instance variables
-var X, Y int
+import (
+    "log"
+    "math/rand"
+    "sync"
+    "time"
+)
 
-// package level functions serve as methods for a package-as-a-singleton
-func F() int {
-    return Y - X
+var (
+    instance string
+    once     sync.Once // initialize instance with once.Do
+)
+
+func claim(color string, w *sync.WaitGroup) {
+    time.Sleep(time.Duration(rand.Intn(1e8))) // hesitate up to .1 sec
+    log.Println("trying to claim", color)
+    once.Do(func() { instance = color })
+    log.Printf("tried %s. instance: %s", color, instance)
+    w.Done()
+}
+
+func main() {
+    rand.Seed(time.Now().Unix())
+    var w sync.WaitGroup
+    w.Add(2)
+    go claim("red", &w) // these two attempts run concurrently
+    go claim("blue", &w)
+    w.Wait()
+    log.Println("after trying both, instance =", instance)
 }

Task/Singleton/Go/singleton-2.go

@@ -1,14 +1,14 @@
-package main
+package singlep
 
-import (
-    "fmt"
-    "singlep"
-)
+// package level data declarations serve as singleton instance variables
+var X, Y int
 
-func main() {
-    // dot selector syntax references package variables and functions
-    singlep.X = 2
-    singlep.Y = 3
-    fmt.Println(singlep.X, singlep.Y)
-    fmt.Println(singlep.F())
+// package level initialization can serve as constructor code
+func init() {
+    X, Y = 2, 3
+}
+
+// package level functions serve as methods for a package-as-a-singleton
+func F() int {
+    return Y - X
 }

Task/Singleton/Go/singleton-3.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+    "fmt"
+    "singlep"
+)
+
+func main() {
+    // dot selector syntax references package variables and functions
+    fmt.Println(singlep.X, singlep.Y)
+    fmt.Println(singlep.F())
+}

Task/Singleton/Go/singleton-4.go

@@ -0,0 +1,24 @@
+package single
+
+import (
+    "log"
+    "sync"
+)
+
+var (
+    color string
+    once  sync.Once
+)
+
+func Color() string {
+    if color == "" {
+        panic("color not initialized")
+    }
+    return color
+}
+
+func SetColor(c string) {
+    log.Println("color initialization")
+    once.Do(func() { color = c })
+    log.Println("color initialized to", color)
+}

Task/Singleton/Go/singleton-5.go

@@ -0,0 +1,12 @@
+package red
+
+import (
+    "log"
+
+    "single"
+)
+
+func SetColor() {
+    log.Println("trying to set red")
+    single.SetColor("red")
+}

Task/Singleton/Go/singleton-6.go

@@ -0,0 +1,12 @@
+package blue
+
+import (
+    "log"
+
+    "single"
+)
+
+func SetColor() {
+    log.Println("trying to set blue")
+    single.SetColor("blue")
+}

Task/Singleton/Go/singleton-7.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+    "log"
+    "math/rand"
+    "time"
+
+    "blue"
+    "red"
+    "single"
+)
+
+func main() {
+    rand.Seed(time.Now().Unix())
+    switch rand.Intn(3) {
+    case 1:
+        red.SetColor()
+        blue.SetColor()
+    case 2:
+        blue.SetColor()
+        red.SetColor()
+    }
+    log.Println(single.Color())
+}