Structs and Interfaces

Go Error Handling: errors.Is and errors.As

Errors are values in Go. Return them, add context with %w, and check them the right way.

Intermediate
4 min read
1 runnable example

What you'll learn

Returning errors
Wrapping with %w
errors.Is and errors.As

Run code →

findUser(id)

findUser(1)"Ada", nil

findUser(7)wrapped ErrNotFound

findUser(-1)*ValidationError

Check the error, then continue

Go has no exceptions for ordinary failures. A function that can fail returns an error as its last value, and the caller checks whether it is nil.

ErrNotFound is a fixed error value that callers can compare against. %w wraps it with more context, and errors.Is still finds it inside the wrapped error. ValidationError is an error type with its own data, and errors.As extracts it.

Key points

nil means success.
%w wraps an error with more context.
errors.Is finds an error inside a wrapped one.

Runnable Go example

Run code →

main.go

package main

import (
	"errors"
	"fmt"
)

var ErrNotFound = errors.New("user not found")

type ValidationError struct {
	Field string
}

func (e *ValidationError) Error() string {
	return "invalid " + e.Field
}

func findUser(id int) (string, error) {
	if id <= 0 {
		return "", &ValidationError{Field: "id"}
	}
	if id != 1 {
		return "", fmt.Errorf("find user %d: %w", id, ErrNotFound)
	}
	return "Ada", nil
}

func main() {
	for _, id := range []int{1, 7, -1} {
		name, err := findUser(id)
		var validation *ValidationError
		switch {
		case err == nil:
			fmt.Println("found", name)
		case errors.Is(err, ErrNotFound):
			fmt.Println("missing:", err)
		case errors.As(err, &validation):
			fmt.Println("bad input:", validation.Field)
		}
	}
}

Get a Run button for this example Result of running the code

found Ada
missing: find user 7: user not found
bad input: id

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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