Initial data commit

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)
+}