Package: errors

package errors Import Path errors (on go.dev) Dependency Relation imports 2 packages, and imported by 145 packages

Involved Source Files d errors.go 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]. [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 [*io/fs.PathError]. join.go wrap.go

Code Examples 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) } } As 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) } } } Is 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) } } } Join 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") } } New package main import ( "errors" "fmt" ) func main() { err := errors.New("emit macho dwarf: elf header corrupted") if err != nil { fmt.Print(err) } } New_errorf 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) } } Unwrap 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)) }

Package-Level Functions (total 5)

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 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.

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 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.

func Join(errs ...error) error 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.

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.

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 only calls a method of the form "Unwrap() error". In particular Unwrap does not unwrap errors returned by [Join].

Package-Level Variables (only one)

var ErrUnsupported error 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.