Package errors
Package errors implements functions to manipulate errors.
The New function creates errors whose only content is a text message.
The Unwrap, Is and As functions work on errors that may wrap other errors.
An error wraps another error if its type has the method
Unwrap() error
If e.Unwrap() returns a non-nil error w, then we say that e wraps w.
Unwrap unpacks wrapped errors. If its argument's type has an
Unwrap method, it calls the method once. Otherwise, it returns nil.
A simple way to create wrapped errors is to call fmt.Errorf and apply the %w verb
to the error argument:
errors.Unwrap(fmt.Errorf("... %w ...", ..., err, ...))
returns err.
Is unwraps its first argument sequentially 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 unwraps its first argument sequentially 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 chain that matches target, and if one is found, sets
target to that error value and returns true. Otherwise, it returns false.
The chain consists of err itself followed by the sequence of errors obtained by
repeatedly calling Unwrap.
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 chain matches target.
The chain consists of err itself followed by the sequence of errors obtained by
repeatedly calling Unwrap.
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 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.
▾ Example
Code:
err1 := errors.New("error1")
err2 := fmt.Errorf("error2: [%w]", err1)
fmt.Println(err2)
fmt.Println(errors.Unwrap(err2))