Package errors
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).
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 fs.ErrExist.
As examines the tree of its first argument looking for an error that can be
assigned to its second argument, which must be a pointer. If it succeeds, it
performs the assignment and returns true. Otherwise, it returns false. The form
var perr *fs.PathError
if errors.As(err, &perr) {
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 *fs.PathError.
▾ Example
Code:
package errors_test
import (
"fmt"
"time"
)
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 Example() {
if err := oops(); err != nil {
fmt.Println(err)
}
}
func As
¶
1.13
func As(err error, target any) bool
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.
The tree consists of err itself, followed by the errors obtained by repeatedly
calling Unwrap. 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(interface{}) 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
Code:
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
func Is
¶
1.13
func Is(err, target error) bool
Is reports whether any error in err's tree matches target.
The tree consists of err itself, followed by the errors obtained by repeatedly
calling Unwrap. 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
Code:
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
func Join
¶
1.20
func Join(errs ...error) error
Join returns an error that wraps the given errors.
Any nil error values are discarded.
Join returns nil if errs contains no non-nil values.
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.
▾ Example
Code:
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")
}
Output:
err1
err2
err is err1
err is err2
func New(text string) error
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
Code:
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.
Code:
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
func Unwrap(err error) error
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 returns nil if the Unwrap method returns []error.
▾ Example
Code:
err1 := errors.New("error1")
err2 := fmt.Errorf("error2: [%w]", err1)
fmt.Println(err2)
fmt.Println(errors.Unwrap(err2))