Go 2 error values today

Last year, the Go team introduced two draft designs related to errors in Go: one introducing new syntax and the other focussing on improved library support. With the help of community feedback, we have turned the latter into a proposal. An implementation, which is planned to be part of Go 1.13, is now in the Go source tree for evaluation. In this talk, I'll explain its design and how to use the new functionality today.

Transcript

1. Go 2 Error Values Today dotGo 2019 Marcel van Lohuizen Google mpvl@golang.org Gopher by Renée French
2. Previously at a theatre near you…
3. End users Users of errors Developers Programs
4. informative, succinct,
 possibly localized End users "ill-behaved gophers abound: hole in fence” "overal slecht gemanierde gophers: gat in hek”
5. more detail Developers ill-behaved gophers abound:
 main.main
 /home/gopher/src/den/main.go:10
 - hole in fence:
 fence.Check
 /home/gopher/src/den/fence/state.go:9
6. Programs strings.Contains(err.Error(), "not exists") err == io.EOF p, ok := err.(*PathError) error inspection
7. Error values in core Today
8. fmt.Errorf Common patterns in the core libraries if err != nil { return fmt.Errorf("whoops: %v", err) }
9. fmt.Errorf discards information fmt.Errorf
10. Custom error type Common patterns in the core libraries return &PathError{"open", filename, err} In core p, ok := err.(*PathError) if p.Err == ... if os.IsPermission(err) { Is it an “exists” error? With helper
11. Lack of standard for error wrapping leads to awkward code Custom error types
12. github.com/pkg/errors Enter the community gopkg.in/errgo.v2/errors github.com/spacemonkeygo/errors go.uber.org/multierr upspin.io/errors github.com/juju/errors github.com/go-errors/errors github.com/hashicorp/errwrap
13. Poor interoperability between the errors of these different packages fmt.Errorf
14. Body Level One Body Level Two Body Level Three Body Level Four Title Text Where from here? Photo by Javier Allegue Barros on Unsplash
15. Goal custom errors == good thing
16. Goal need a standard and interoperability • inspection • details • localizability
17. Body Level One Body Level Two Body Level Three Body Level Four Title Text Go 1.13 Photo by Proposal by Nick Youngson CC BY-SA 3.0 ImageCreator on Unsplash
18. Inspection Proposal #29934
19. Error inspection: Wrapper Programmatic analysis package errors type Wrapper interface { Unwrap() error } Optional interface func (e *PathError) Unwrap() error { return e.Err } Example implementation
20. Error Inspection: errors.Is Programmatic analysis // Is reports whether err or any of the // errors in its chain is equal to target. func Is(err, target error) bool API for sentinel comparison // was if err == io.EOF { if errors.Is(err, io.EOF) { Example usage
21. func Cause(err error) error This is not:
22. Error inspection Programmatic analysis if errors.Is(err, os.ErrPermission) { … } Now if os.IsPermission(err) { … } Before
23. Error inspection Programmatic analysis if errors.Is(err, net.ErrTimeout) { … } Now if err, ok := err.(net.Error); ok && err.Timeout() { … } Before
24. Error Inspection: errors.As Programmatic analysis func As(err error, target interface{}) bool API for type assertion // was if p, ok := err.(*os.PathError); ok { var p *os.PathError if errors.As(err, &p) { if p := (*os.PathError)(nil); errors.As(err, &p) { Example usage
25. Printing Proposal #29934 Gopher by Renée French
26. Normal versus Detail Common patterns in the core libraries ill-behaved gophers abound:
 main.main
 /home/gopher/src/den/main.go:10
 - hole in fence:
 fence.Check
 /home/gopher/src/den/fence/state.go:9 fmt.Printf("%+v", err) fmt.Printf("%v", err) ill-behaved gophers abound: hole in fence
27. Error printing: Formatter and Printer Formatting package errors type Formatter interface { error FormatError(p Printer) (next error) } type Printer interface { Print(args ...interface{}) Printf(format string, args ...interface{}) Detail() bool } Interfaces
28. Error details Formatting package errors func Caller(skip int) Frame type Frame struct { … } func (f Frame) Format(p *Printer) errors.Frame
29. Error Printing: Example Formatting type PathError struct { Op, Path string Err error frame Frame } func (e *PathError) FormatError(p errors.Printer) error { p.Printf("%s %s", e.Op, e.Path) e.frame.Format(p) // Format calls p.Detail return e.Err } func (e *PathError) Error() string { return fmt.Sprint(e) }
30. Body Level One Body Level Two Body Level Three Body Level Four Title Text Migration Photo by Jan-Niclas Aberle on Unsplash Adapting to the new standard
31. • support Unwrap and make errors.Is work with: - os.ErrPermission - os.ErrExist - os.ErrNotExist - … • add location information through FormatError Core error types
32. • print single stack frame in detailed mode Changes to errors.New and fmt.Errorf
33. •FormatError to return err for 
 fmt.Errorf("oops: %v", err)
 fmt.Errorf("oops: %s", err) •also adds Unwrap for
 fmt.Errorf("oops: %w", err) Changes to fmt.Errorf
34. Announcement new functionality available today: golang.org/x/xerrors
35. import "golang.org/x/xerrors" type GopherError struct { frame xerrors.Frame err error } func (e *GopherError) Format(s fmt.State, verb rune) { xerrors.FormatError(e, s, verb) } func (e *GopherError) FormatError(p xerrors.Printer) error { p.Print("ill-behaving gophers abound") e.frame.Format(p) return err } func (e *GopherError) Error() string { return fmt.Sprint(e) } Using golang.org/x/xerrors
36. Community
37. @mpvl_ Thank you dotGo 2019 Marcel van Lohuizen Google mpvl@golang.org Gophers by Renée French