Add defer.md

Author: NanXiao

SUMMARY.md

@@ -25,6 +25,7 @@
 * [Debugging](posts/debugging.md)
 * [Goroutine](posts/goroutine.md)
 * [Functional literals](posts/functional-literals.md)
+* [defer](posts/defer.md)
 * [error vs errors](posts/error-vs-errors.md)
 * [Send and receive operations on channel](posts/send-and-receive-operations-on-channel.md)
 * [Channel types](posts/channel-types.md)

posts/defer.md

@@ -0,0 +1,76 @@
+# defer
+----
+The `defer` statement is used to postpone a function call executed immediately before the surrounding function returns. The common uses of `defer` include releasing resources (i.e., unlock the mutex, close file handle.), do some tracing(i.e., record the running time of function), etc. E.g., an ordinary accessing global variable exclusively is like this:  
+
+	var mu sync.Mutex
+	var m = make(map[string]int)
+	
+	func lookup(key string) int {
+		mu.Lock()
+		v := m[key]
+		mu.Unlock()
+		return v
+	}
+An equivalent but concise format using `defer` is as follow:  
+	
+	var mu sync.Mutex
+	var m = make(map[string]int)
+	
+	func lookup(key string) int {
+		mu.Lock()
+		defer mu.Unlock()
+		return m[key]
+	}
+You can see this style is more simpler and easier to comprehend.  
+
+The `defer` statements are executed in Last-In-First-Out sequence, which means the functions in latter `defer` statements run before their previous buddies. Check the following example:  
+
+	package main
+	
+	import "fmt"
+	
+	func main()  {
+		defer fmt.Println("First")
+		defer fmt.Println("Last")
+	}
+
+The running result is here:  
+
+	Last
+	First
+
+Although the function in `defer` statement runs very late, the parameters of the function are evaluated when the `defer` statement is executed.  
+
+	package main
+	
+	import "fmt"
+	
+	func main()  {
+		i := 10
+		defer fmt.Println(i)
+		i = 100
+	}
+The running result is here: 
+
+	10
+Besides, if the deferred function is a function literal, it can also modify the return value:  
+
+	package main
+	
+	import "fmt"
+	
+	func modify() (result int) {
+		defer func(){result = 1000}()
+		return 100
+	}
+	
+	func main()  {
+		fmt.Println(modify())
+	}  
+
+The value printed is `1000`, not `100`.  
+
+References:  
+[The Go Programming Language](http://www.gopl.io/);  
+[Defer, Panic, and Recover](https://blog.golang.org/defer-panic-and-recover).
+