Overview

The recover function  returns the value which was passed to the panic function. Therefore it is a good practice to check the return value of the recover function. If the return value is non nil then panic did not happen and recover function was not called with the panic.

Example

Let’s see a program to fully understand it

package main

import "fmt"

func main() {

	a := []string{"a", "b"}
	checkAndPrint(a, 2)
	fmt.Println("Exiting normally")
}

func checkAndPrint(a []string, index int) {
	defer handleOutOfBounds()
	if index > (len(a) - 1) {
		panic("Out of bound access for slice")
	}
	fmt.Println(a[index])
}

func handleOutOfBounds() {
	if r := recover(); r != nil {
		fmt.Println("Recovering from panic:", r)
	}
}

Output

Recovering from panic: Out of bound access for slice
Exiting normally

In the above program we have a function checkAndPrint which checks and prints slice element at an index passed in the argument. If the index passed is greater than the length of the array then the program panics.  We have added a defer function named handleOutIfBounds as well at the start of the function checkAndPrint.   This function  contains  the  call to recover function  as below

if r := recover(); r != nil {
    fmt.Println("Recovering from panic:", r)
}

The recover function will catch the panic and we can also print the message from the panic. 

Recovering from panic: Out of bound access for slice

After the recover function the program continues and the control returns to the called function which is  main here. That is why we get output as

Exiting normally

The recover function returns the value which was passed to the panic function. That is why we have below code in the  defer function handleOutofBounds

if r := recover(); r != nil 

Here if r is nil then panic did not happened. So if there is no panic then call to recover will return nil