Go 2 Error Values Today dotGo 2019 Marcel van Lohuizen Google [email protected] Gopher by Renée French 

Previously at a theatre near you… 

End users Users of errors Developers Programs 

informative, succinct,
 possibly localized End users "ill-behaved gophers abound: hole in fence” "overal slecht gemanierde gophers: gat in hek” 

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 

Programs strings.Contains(err.Error(), "not exists") err == io.EOF p, ok := err.(*PathError) error inspection 

Error values in core Today 

fmt.Errorf Common patterns in the core libraries if err != nil { return fmt.Errorf("whoops: %v", err) } 

fmt.Errorf discards information fmt.Errorf 

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 

Lack of standard for error wrapping leads to awkward code Custom error types 

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 

Poor interoperability between the errors of these different packages fmt.Errorf 

Body Level One Body Level Two Body Level Three Body Level Four Title Text Where from here? Photo by Javier Allegue Barros on Unsplash 

Goal custom errors == good thing 

Goal need a standard and interoperability • inspection • details • localizability 

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 

Inspection Proposal #29934 

Error inspection: Wrapper Programmatic analysis package errors type Wrapper interface { Unwrap() error } Optional interface func (e *PathError) Unwrap() error { return e.Err } Example implementation 

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 

func Cause(err error) error This is not: 

Error inspection Programmatic analysis if errors.Is(err, os.ErrPermission) { … } Now if os.IsPermission(err) { … } Before 

Error inspection Programmatic analysis if errors.Is(err, net.ErrTimeout) { … } Now if err, ok := err.(net.Error); ok && err.Timeout() { … } Before 

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 

Printing Proposal #29934 Gopher by Renée French 

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 

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 

Error details Formatting package errors func Caller(skip int) Frame type Frame struct { … } func (f Frame) Format(p *Printer) errors.Frame 

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) } 

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 

• support Unwrap and make errors.Is work with: - os.ErrPermission - os.ErrExist - os.ErrNotExist - … • add location information through FormatError Core error types 

• print single stack frame in detailed mode Changes to errors.New and fmt.Errorf 

•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 

Announcement new functionality available today: golang.org/x/xerrors 

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 

Community 

@mpvl_ Thank you dotGo 2019 Marcel van Lohuizen Google [email protected] Gophers by Renée French