Go Errors
An error is a value that reports a problem. A function returns an error value beside its normal result.
Return an Error
Divide returns an int and an error. A zero divisor gives the error path.
package main
import "fmt"
import "errors"
func divide(a int, b int) (int, error) {
if b == 0 {
return 0, errors.New("cannot divide 10 by zero")
}
return a / b, nil
}
func main() {
_, err := divide(10, 0)
if err != nil {
fmt.Println(err)
}
}The Success Path
A divisor of 2 is fine, so the error comes back nil. The quotient then prints.
package main
import "fmt"
import "errors"
func divide(a int, b int) (int, error) {
if b == 0 {
return 0, errors.New("cannot divide 10 by zero")
}
return a / b, nil
}
func main() {
q, err := divide(10, 2)
if err == nil {
fmt.Println(q)
}
}An Error From a Package
Atoi reads text as a number, and the text abc is not a number. Its error prints in full.
package main
import "fmt"
import "strconv"
func main() {
n, err := strconv.Atoi("abc")
if err != nil {
fmt.Println(err)
} else {
fmt.Println(n)
}
}A Runtime Panic
The slice has length 3, so index 5 does not exist. The program panics and ends.
package main
import "fmt"
func main() {
nums := []int{1, 2, 3}
fmt.Println(nums[5])
}Defer Runs Last In, First Out
Two calls are deferred in main. They run at the end of main, in reverse order.
package main
import "fmt"
func main() {
defer fmt.Println("first deferred")
defer fmt.Println("second deferred")
fmt.Println("main body")
}Example
package main
import "fmt"
import "errors"
func divide(a int, b int) (int, error) {
if b == 0 {
return 0, errors.New("cannot divide 10 by zero")
}
return a / b, nil
}
func main() {
_, err := divide(10, 0)
if err != nil {
fmt.Println(err)
}
q, err := divide(20, 4)
if err == nil {
fmt.Println(q)
}
fmt.Println("Program continues.")
}Key points
- Error: a value that reports a problem.
- A function returns an error beside its normal result.
- A nil error means no problem happened.
- Check the error value before trusting the other result.
- Use errors.New to build an error value from a message.
- Panic: a runtime failure that ends the program.
- Defer schedules a call to run at the end of the current function.
- Deferred calls run in last-in, first-out order.
In this example, the first call divides 10 by 0 and returns an error. Main prints that error and keeps going. The second call divides 20 by 4 and returns a nil error. Main prints the quotient 5. The last line shows the program running on after an error.
Try it Yourself
Edit the code, then press Run.
Running is simulated · Go Playground
Exercise
A function returns a result and an error, and the error is nil. What does nil mean here?