Golang Recover Built In: Handle Errors, Panics

Use defer functions and the recover built-in to handle errors. Go uses recover, not exception-handling.

Recover. Failure is everywhere. A method might work when we test it. But it is just waiting to fail at the worst moment. Go provides ways to handle errors.Built-Ins

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. Let us begin. This program introduces a hopeless method called "divideByZero." It causes an error to occur whenever it is run.

Defer: 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.

Golang program that uses recover, defer func 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() } Output 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.

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

Golang program that uses panic, defer and recover 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() } Output 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.

Golang program that causes type string error 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") } Output # command-line-arguments C:\programs\file.go:11: type string is not an expression

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).

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Golang Recover Built In: Handle Errors, Panics

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