Passing parameters to function closure

Question

I'm trying to understand the difference in Go between creating an anonymous function which takes a parameter, versus having that function act as a closure. Here is an example of the difference.

With parameter:

func main() {   done := make(chan bool, 1)   go func(c chan bool) {     time.Sleep(50 * time.Millisecond)     c <- true   }(done)   <-done } 

As closure:

func main() {   done := make(chan bool, 1)   go func() {     time.Sleep(50 * time.Millisecond)     done <- true   }()   <-done } 

My question is, when is the first form better than the second? Would you ever use a parameter for this kind of thing? The only time I can see the first form being useful is when returning a func(x, y) from another function.

Answer 1

The difference between using a closure vs using a function parameter has to do with sharing the same variable vs getting a copy of the value. Consider these two examples below.

In the Closure all function calls will use the value stored in i. This value will most likely already reach 3 before any of the goroutines has had time to print it's value.

In the Parameter example each function call will get passed a copy of the value of i when the call was made, thus giving us the result we more likely wanted:

Closure:

for i := 0; i < 3; i++ {     go func() {         fmt.Println(i)     }() } 

Result:

3
3
3

Parameter:

for i := 0; i < 3; i++ {     go func(v int) {         fmt.Println(v)     }(i) } 

Result:

0
1
2

Playground: http://play.golang.org/p/T5rHrIKrQv