go-tour: add defer slides

LGTM=adg
R=adg, campoy
CC=golang-codereviews
https://codereview.appspot.com/110520043

Committer: Andrew Gerrand <[email protected]>

content/flowcontrol.article

@@ -108,6 +108,26 @@ This construct can be a clean way to write long if-then-else chains.
 
 .play prog/tour/switch-with-no-condition.go
 
+* Defer
+
+A defer statement defers the execution of a function until the surrounding
+function returns.
+
+The deferred call's arguments are evaluated immediately, but the function call
+is not executed until the surrounding function returns.
+
+.play prog/tour/defer.go
+
+* Stacking defers
+
+Deferred function calls are pushed onto a stack. When a function returns, its
+deferred calls are executed in last-in-first-out order.
+
+To learn more about defer statements read this
+[[http://blog.golang.org/defer-panic-and-recover][blog post]].
+
+.play prog/tour/defer-multi.go
+
 * Congratulations!
 
 You finished this lesson!

content/prog/tour/defer-multi.go

@@ -0,0 +1,13 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("counting")
+
+	for i := 0; i < 10; i++ {
+		defer fmt.Println(i)
+	}
+
+	fmt.Println("done")
+}

content/prog/tour/defer.go

@@ -0,0 +1,9 @@
+package main
+
+import "fmt"
+
+func main() {
+	defer fmt.Println("world")
+
+	fmt.Println("hello")
+}