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)
 }