Add decorator pattern

Author: tmrts

README.md

@@ -28,7 +28,7 @@ __Structural Patterns__:
 | [Adapter](adapter.go) | Adapts otherwise incompatible interfaces to work together by adapting one to the other |
 | [Bridge](bridge.go) | Decouples an interface from its implementation so that the two can vary independently |
 | [Composite](composite.go) | Encapsulates and provides access to a number of different objects |
-| [Decorator](decorator.go) | Adds behavior to an object, statically or dynamically |
+| [Decorator](structural/decorator.md) | Adds behavior to an object, statically or dynamically |
 | [Facade](facade.go) | Uses one class as an API to a number of others |
 | [Flyweight](flyweight.go) | Reuses existing instances of objects with similar/identical state to minimize resource usage |
 | [Model View Controller](mvc.go) | Divides an app into three interconnected parts to separate internal representation from presentation to user |

decorator.go

@@ -0,0 +1,41 @@
+# Decorator Pattern
+Decorator structural pattern allows extending the function of an existing object dynamically without altering its internals.
+
+Decorators provide a flexible method to extend functionality of objects.
+
+## Implementation
+`LogDecorate` decorates a function with the signature `func(int) int` that 
+manipulates integers and adds input/output logging capabilities.
+
+```go
+type Object func(int) int
+
+func LogDecorate(fn Object) Object {
+	return func(n int) int {
+		log.Println("Starting the execution with the integer", n)
+
+		result := fn(n)
+
+		log.Println("Execution is completed with the result", result)
+
+        return result
+	}
+}
+```
+
+## Usage
+```go
+func Double(n int) int {
+    return n * 2
+}
+
+f := LogDecorate(Double)
+
+f(5)
+// Starting execution with the integer 5
+// Execution is completed with the result 10
+```
+
+## Rules of Thumb
+- Unlike Adapter pattern, the object to be decorated is obtained by **injection**.
+- Decorators should not alter the interface of an object.