Error Handling

Interface

Golang provides an interface type error:

type error interface {
    Error() string
}

Any structure that implements the Error() method belongs to the error type.

Creating Errors

Golang provides several ways to create an error.

We can use errors.New() or fmt.Errorf() to create errors.

Example

package main

import (
	"errors"
	"fmt"
)

func main() {
    // Create error using New
	err1 := errors.New("first error")

    // Create error using fmt
	err2 := fmt.Errorf("second %s", "error")

	fmt.Println(err1, err2)
}

errors.Join

In Go 1.20, errors.Join was introduced to combine multiple errors into one array.

Example

package main

import (
	"errors"
	"fmt"
)

func main() {
	// Create error using New
	err1 := errors.New("first error")

	// Create error using fmt
	err2 := fmt.Errorf("second %s", "error")

	// Use join to combine multiple errors
	err3 := errors.Join(err1, err2)

	fmt.Println(err1, err2) 
	// Output: first error second error
	
	fmt.Println(err3) 
	// Output: 
	// first error
	// second error
}

errors.Is

We can use errors.Is() to determine whether the current error contains the target type of error.

Example

package main

import (
	"errors"
	"fmt"
)

func main() {
	// Create error using New
	err1 := errors.New("first error")

	// Create error using fmt
	err2 := fmt.Errorf("second %s", "error")

	// Use join to combine multiple errors
	err3 := errors.Join(err1, err2)

	fmt.Println(err1, err2) 
	// Output: first error second error
	
	fmt.Println(err3) 
	// Output: 
	// first error
	// second error

	// Use errors.Is() to determine whether the error is the target error. Since err3 contains err1, it is true.
	fmt.Println(errors.Is(err1, err2))
	// Output: false
	fmt.Println(errors.Is(err3, err1))
	// Output: true
}

errors.As

We can use errors.As() to assign the first error in the error that matches the target type to the target object.

Example

package main

import (
	"errors"
	"fmt"
)

type NormalErr struct {
	e string
}

func (t NormalErr) Error() string {
	return t.e
}

func main() {
	// Create error using New
	err1 := errors.New("first error")

	// Create T
	err2 := NormalErr{"second error"}

	// Use join to combine multiple errors
	err3 := errors.Join(err1, err2)

	var err4 NormalErr

	errors.As(err3, &err4)

	fmt.Println(err4)
	// Output： second error
}