import "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.
A simple way to create wrapped errors is to call fmt.Errorf and apply the %w verb to the error argument:
fmt.Errorf("... %w ...", ..., err, ...).Unwrap()
returns err.
Unwrap unpacks wrapped errors. If its argument's type has an Unwrap method, it calls the method once. Otherwise, it returns nil.
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, os.ErrExist)
is preferable to
if err == os.ErrExist
because the former will succeed if err wraps os.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 *os.PathError
if errors.As(err, &perr) {
fmt.Println(perr.Path)
}
is preferable to
if perr, ok := err.(*os.PathError); ok {
fmt.Println(perr.Path)
}
because the former will succeed if err wraps an *os.PathError.
Code:
package errors_test
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 Example() {
if err := oops(); err != nil {
fmt.Println(err)
}
// Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
}
Output:
1989-03-15 22:30:00 +0000 UTC: the file system has gone away
func As(err error, target interface{}) bool
As finds the first error in err's chain that matches target, and if so, sets target to that error value and returns true.
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.
As will panic if target is not a non-nil pointer to either a type that implements error, or to any interface type. As returns false if err is nil.
Code:
if _, err := os.Open("non-existing"); err != nil {
var pathError *os.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(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.
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.
Code:
err := errors.New("emit macho dwarf: elf header corrupted")
if err != nil {
fmt.Print(err)
}
Output:
emit macho dwarf: elf header corrupted
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.