Recover

Go provides ways to handle errors. Failure is everywhere—a method might work when we test it, but other conditions might cause it to panic later.

With recover, a built-in method, we get errors and can test them. We must specify a call to recover() in a "defer" method. The Go runtime calls these methods when an error occurs.

First example

This program introduces an error-prone method called "divideByZero." It causes an error to occur whenever it is run.

Detail In divideByZero we specify a nested "defer" method. In this func we assign err to the result of recover().

And If err is not nil, we take an action that can repair the program's state and keep it from terminating.

Tip In this program, if we remove the divide by zero error, the value of "dog" will be printed. And the "defer" func will never be called.

package main

import "fmt"

func divideByZero() {
    // Use this deferred function to handle errors.
    defer func() {
        if err := recover(); err != nil {
            fmt.Println("HERE")
            fmt.Println(err)
            fmt.Println(0)
        }
    }()
    // Cause an error.
    // ... Go will run the defer func above.
    cat := 0
    dog := 10 / cat
    fmt.Println(dog)
}

func main() {
    // Create a divide by zero error and handle it.
    divideByZero()
}HERE
runtime error: integer divide by zero
0

Panic

This built-in method causes an error to occur. We can handle this error in a defer method that tests the result of recover().

Tip When panic is called, the stack is unwound. And each method is tested for a defer method.

Detail The recover() method returns panic's error. In this program, the WRONG message is provided.

package main

import "fmt"

func explode() {
    // Cause a panic.
    panic("WRONG")
}

func main() {
    // Handle errors in defer func with recover.
    defer func() {
        if err := recover(); err != nil {
            // Handle our error.
            fmt.Println("FIX")
            fmt.Println("ERR", err)
        }
    }()
    // This causes an error.
    explode()
}FIX
ERR WRONG

Compile-time error

Sometimes a Go developer uses the wrong variable name. For example, I wrote a program that used "string" instead of the identifier "name."

Result The "type string is not an expression" error occurs. Use the correct identifier "name" for a working program.

package main

import "fmt"

func mistake(name string) {
    // We put the wrong variable name.
    // ... Use "name" not string.
    fmt.Println(string)
}

func main() {
    mistake("cat")
}
# command-line-arguments
C:\programs\file.go:11: type string is not an expression

Performance, defer

The defer keyword has a performance impact. It saves the method call to be called after all returns or panics are executed in a method.

Version 1 This version of the code uses defer to call the Add() function once a return statement is reached in Test.

Version 2 This code does not use defer. It simply calls Add() before each return statement, and does not worry about panics.

Result The code that avoids defer is significantly faster. If panics do not need to be supported, avoiding defer is worth considering.

package main

import (
    "fmt"
    "time"
)

type Data struct {
    code int
}

func Add(data *Data) {
    data.code++
}

func Test(value int, data *Data) int {
    defer Add(data)
    if value == 2 {
        return 3
    }
    return 4
}

func TestNoDefer(value int, data *Data) int {
    if value == 2 {
        Add(data)
        return 3
    }
    Add(data)
    return 4
}

func main() {
    data := new(Data)
    t0 := time.Now()
    // Version 1: use defer.
    for i := 0; i < 200000000; i++ {
        Test(2, data)
    }
    t1 := time.Now()
    // Version 2: do not use defer.
    for i := 0; i < 200000000; i++ {
        TestNoDefer(2, data)
    }
    fmt.Println(t1.Sub(t0))
    fmt.Println(time.Now().Sub(t1))
}646.339875ms  defer Add
423.013125ms  Add

Defer guidelines

If a method is simple, and is known to be reliable, defer is probably not needed and will just slow down the program. If it has file handling, defer is probably worth using.

A summary

With recover, defer and panic we handle errors in Go programs. This is an alternative to exception handling (like try and catch in other languages).