Documentation
¶
Overview ¶
Package errors implements functions to manipulate errors.
The New function creates errors whose only content is a text message.
An error e wraps another error if e's type has one of the methods
Unwrap() error Unwrap() []error
If e.Unwrap() returns a non-nil error w or a slice containing w, then we say that e wraps w. A nil error returned from e.Unwrap() indicates that e does not wrap any error. It is invalid for an Unwrap method to return an []error containing a nil error value.
An easy way to create wrapped errors is to call fmt.Errorf and apply the %w verb to the error argument:
wrapsErr := fmt.Errorf("... %w ...", ..., err, ...)
Successive unwrapping of an error creates a tree. The Is and As functions inspect an error's tree by examining first the error itself followed by the tree of each of its children in turn (pre-order, depth-first traversal).
See https://go.dev/blog/go1.13-errors for a deeper discussion of the philosophy of wrapping and when to wrap.
Is examines the tree of its first argument looking for an error that matches the second. It reports whether it finds a match. It should be used in preference to simple equality checks:
if errors.Is(err, fs.ErrExist)
is preferable to
if err == fs.ErrExist
because the former will succeed if err wraps io/fs.ErrExist.
AsType examines the tree of its argument looking for an error whose type matches its type argument. If it succeeds, it returns the corresponding value of that type and true. Otherwise, it returns the zero value of that type and false. The form
if perr, ok := errors.AsType[*fs.PathError](err); ok {
fmt.Println(perr.Path)
}
is preferable to
if perr, ok := err.(*fs.PathError); ok {
fmt.Println(perr.Path)
}
because the former will succeed if err wraps an *io/fs.PathError.
Example ¶
package main
import (
"fmt"
"time"
)
// MyError is an error implementation that includes a time and message.
type MyError struct {
When time.Time
What string
}
func (e MyError) Error() string {
return fmt.Sprintf("%v: %v", e.When, e.What)
}
func oops() error {
return MyError{
time.Date(1989, 3, 15, 22, 30, 0, 0, time.UTC),
"the file system has gone away",
}
}
func main() {
if err := oops(); err != nil {
fmt.Println(err)
}
}
Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
Index ¶
Examples ¶
Constants ¶
This section is empty.
Variables ¶
var ErrUnsupported = New("unsupported operation")
ErrUnsupported indicates that a requested operation cannot be performed, because it is unsupported. For example, a call to os.Link when using a file system that does not support hard links.
Functions and methods should not return this error but should instead return an error including appropriate context that satisfies
errors.Is(err, errors.ErrUnsupported)
either by directly wrapping ErrUnsupported or by implementing an Is method.
Functions and methods should document the cases in which an error wrapping this will be returned.
Functions ¶
func As ¶ added in go1.13
As finds the first error in err's tree that matches target, and if one is found, sets target to that error value and returns true. Otherwise, it returns false.
For most uses, prefer AsType. As is equivalent to AsType but sets its target argument rather than returning the matching error and doesn't require its target argument to implement error.
The tree consists of err itself, followed by the errors obtained by repeatedly calling its Unwrap() error or Unwrap() []error method. When err wraps multiple errors, As examines err followed by a depth-first traversal of its children.
An error matches target if the error's concrete value is assignable to the value pointed to by target, or if the error has a method As(any) bool such that As(target) returns true. In the latter case, the As method is responsible for setting target.
An error type might provide an As method so it can be treated as if it were a different error type.
As panics if target is not a non-nil pointer to either a type that implements error, or to any interface type.
Example ¶
package main
import (
"errors"
"fmt"
"io/fs"
"os"
)
func main() {
if _, err := os.Open("non-existing"); err != nil {
var pathError *fs.PathError
if errors.As(err, &pathError) {
fmt.Println("Failed at path:", pathError.Path)
} else {
fmt.Println(err)
}
}
}
Output: Failed at path: non-existing
Example (Custom_match) ¶
Custom errors can implement a method "As(any) bool" to match against other error types, overriding the default matching of errors.As.
package main
import (
"errors"
"fmt"
"io/fs"
)
type MyAsError struct {
err string
}
func (e MyAsError) Error() string {
return e.err
}
func (e MyAsError) As(target any) bool {
pe, ok := target.(**fs.PathError)
if !ok {
return false
}
*pe = &fs.PathError{
Op: "custom",
Path: "/",
Err: errors.New(e.err),
}
return true
}
func main() {
var err error = MyAsError{"an error"}
fmt.Println("Error:", err)
fmt.Printf("TypeOf err: %T\n", err)
var pathError *fs.PathError
ok := errors.As(err, &pathError)
fmt.Println("Error as fs.PathError:", ok)
fmt.Println("fs.PathError:", pathError)
}
Output: Error: an error TypeOf err: errors_test.MyAsError Error as fs.PathError: true fs.PathError: custom /: an error
func AsType ¶ added in go1.26.0
AsType finds the first error in err's tree that matches the type E, and if one is found, returns that error value and true. Otherwise, it returns the zero value of E and false.
The tree consists of err itself, followed by the errors obtained by repeatedly calling its Unwrap() error or Unwrap() []error method. When err wraps multiple errors, AsType examines err followed by a depth-first traversal of its children.
An error err matches the type E if the type assertion err.(E) holds, or if the error has a method As(any) bool such that err.As(target) returns true when target is a non-nil *E. In the latter case, the As method is responsible for setting target.
Example ¶
package main
import (
"errors"
"fmt"
"io/fs"
"os"
)
func main() {
if _, err := os.Open("non-existing"); err != nil {
if pathError, ok := errors.AsType[*fs.PathError](err); ok {
fmt.Println("Failed at path:", pathError.Path)
} else {
fmt.Println(err)
}
}
}
Output: Failed at path: non-existing
Example (Custom_match) ¶
Custom errors can implement a method "As(any) bool" to match against other error types, overriding the default matching of errors.AsType.
package main
import (
"errors"
"fmt"
"io/fs"
)
type MyAsError struct {
err string
}
func (e MyAsError) Error() string {
return e.err
}
func (e MyAsError) As(target any) bool {
pe, ok := target.(**fs.PathError)
if !ok {
return false
}
*pe = &fs.PathError{
Op: "custom",
Path: "/",
Err: errors.New(e.err),
}
return true
}
func main() {
var err error = MyAsError{"an error"}
fmt.Println("Error:", err)
fmt.Printf("TypeOf err: %T\n", err)
pathError, ok := errors.AsType[*fs.PathError](err)
fmt.Println("Error as fs.PathError:", ok)
fmt.Println("fs.PathError:", pathError)
}
Output: Error: an error TypeOf err: errors_test.MyAsError Error as fs.PathError: true fs.PathError: custom /: an error
func Is ¶ added in go1.13
Is reports whether any error in err's tree matches target. The target must be comparable.
The tree consists of err itself, followed by the errors obtained by repeatedly calling its Unwrap() error or Unwrap() []error method. When err wraps multiple errors, Is examines err followed by a depth-first traversal of its children.
An error is considered to match a target if it is equal to that target or if it implements a method Is(error) bool such that Is(target) returns true.
An error type might provide an Is method so it can be treated as equivalent to an existing error. For example, if MyError defines
func (m MyError) Is(target error) bool { return target == fs.ErrExist }
then Is(MyError{}, fs.ErrExist) returns true. See syscall.Errno.Is for an example in the standard library. An Is method should only shallowly compare err and the target and not call Unwrap on either.
Example ¶
package main
import (
"errors"
"fmt"
"io/fs"
"os"
)
func main() {
if _, err := os.Open("non-existing"); err != nil {
if errors.Is(err, fs.ErrNotExist) {
fmt.Println("file does not exist")
} else {
fmt.Println(err)
}
}
}
Output: file does not exist
Example (Custom_match) ¶
Custom errors can implement a method "Is(error) bool" to match other error values, overriding the default matching of errors.Is.
package main
import (
"errors"
"fmt"
"io/fs"
)
type MyIsError struct {
err string
}
func (e MyIsError) Error() string {
return e.err
}
func (e MyIsError) Is(err error) bool {
return err == fs.ErrPermission
}
func main() {
var err error = MyIsError{"an error"}
fmt.Println("Error equals fs.ErrPermission:", err == fs.ErrPermission)
fmt.Println("Error is fs.ErrPermission:", errors.Is(err, fs.ErrPermission))
}
Output: Error equals fs.ErrPermission: false Error is fs.ErrPermission: true
func Join ¶ added in go1.20
Join returns an error that wraps the given errors. Any nil error values are discarded. Join returns nil if every value in errs is nil. The error formats as the concatenation of the strings obtained by calling the Error method of each element of errs, with a newline between each string.
A non-nil error returned by Join implements the Unwrap() []error method. The errors may be inspected with Is and As.
Example ¶
package main
import (
"errors"
"fmt"
)
func main() {
err1 := errors.New("err1")
err2 := errors.New("err2")
err := errors.Join(err1, err2)
fmt.Println(err)
if errors.Is(err, err1) {
fmt.Println("err is err1")
}
if errors.Is(err, err2) {
fmt.Println("err is err2")
}
fmt.Println(err.(interface{ Unwrap() []error }).Unwrap())
}
Output: err1 err2 err is err1 err is err2 [err1 err2]
func New ¶
New returns an error that formats as the given text. Each call to New returns a distinct error value even if the text is identical.
Example ¶
package main
import (
"errors"
"fmt"
)
func main() {
err := errors.New("emit macho dwarf: elf header corrupted")
if err != nil {
fmt.Print(err)
}
}
Output: emit macho dwarf: elf header corrupted
Example (Errorf) ¶
The fmt package's Errorf function lets us use the package's formatting features to create descriptive error messages.
package main
import (
"fmt"
)
func main() {
const name, id = "bimmler", 17
err := fmt.Errorf("user %q (id %d) not found", name, id)
if err != nil {
fmt.Print(err)
}
}
Output: user "bimmler" (id 17) not found
Example (Unique) ¶
Each call to errors.New returns an unique instance of the error, even if the arguments are the same. To match against errors created by errors.New, declare a sentinel error and reuse it.
package main
import (
"errors"
"fmt"
)
func OopsNew() error {
return errors.New("an error")
}
var ErrSentinel = errors.New("an error")
func OopsSentinel() error {
return ErrSentinel
}
func main() {
err1 := OopsNew()
err2 := OopsNew()
fmt.Println("Errors using distinct errors.New calls:")
fmt.Printf("Is(%q, %q) = %v\n", err1, err2, errors.Is(err1, err2))
err3 := OopsSentinel()
err4 := OopsSentinel()
fmt.Println()
fmt.Println("Errors using a sentinel error:")
fmt.Printf("Is(%q, %q) = %v\n", err3, err4, errors.Is(err3, err4))
}
Output: Errors using distinct errors.New calls: Is("an error", "an error") = false Errors using a sentinel error: Is("an error", "an error") = true
func Unwrap ¶ added in go1.13
Unwrap returns the result of calling the Unwrap method on err, if err's type contains an Unwrap method returning error. Otherwise, Unwrap returns nil.
Unwrap only calls a method of the form "Unwrap() error". In particular Unwrap does not unwrap errors returned by Join.
Example ¶
package main
import (
"errors"
"fmt"
)
func main() {
err1 := errors.New("error1")
err2 := fmt.Errorf("error2: [%w]", err1)
fmt.Println(err2)
fmt.Println(errors.Unwrap(err2))
}
Output: error2: [error1] error1
Types ¶
This section is empty.
Source Files