errors - ActiveState ActiveGo 1.8
...

Package errors

import "github.com/pkg/errors"
Overview
Index
Examples

Overview ▾

Package errors provides simple error handling primitives.

The traditional error handling idiom in Go is roughly akin to 

if err != nil {
        return err
}

which applied recursively up the call stack results in error reports without context or debugging information. The errors package allows programmers to add context to the failure path in their code in a way that does not destroy the original value of the error.

Adding context to an error

The errors.Wrap function returns a new error that adds context to the original error by recording a stack trace at the point Wrap is called, and the supplied message. For example 

_, err := ioutil.ReadAll(r)
if err != nil {
        return errors.Wrap(err, "read failed")
}

If additional control is required the errors.WithStack and errors.WithMessage functions destructure errors.Wrap into its component operations of annotating an error with a stack trace and an a message, respectively.

Retrieving the cause of an error

Using errors.Wrap constructs a stack of errors, adding context to the preceding error. Depending on the nature of the error it may be necessary to reverse the operation of errors.Wrap to retrieve the original error for inspection. Any error value which implements this interface 

type causer interface {
        Cause() error
}

can be inspected by errors.Cause. errors.Cause will recursively retrieve the topmost error which does not implement causer, which is assumed to be the original cause. For example: 

switch err := errors.Cause(err).(type) {
case *MyError:
        // handle specifically
default:
        // unknown error
}

causer interface is not exported by this package, but is considered a part of stable public API.

Formatted printing of errors

All error values returned from this package implement fmt.Formatter and can be formatted by the fmt package. The following verbs are supported 

%s    print the error. If the error has a Cause it will be
      printed recursively
%v    see %s
%+v   extended format. Each Frame of the error's StackTrace will
      be printed in detail.

Retrieving the stack trace of an error or wrapper

New, Errorf, Wrap, and Wrapf record a stack trace at the point they are invoked. This information can be retrieved with the following interface. 

type stackTracer interface {
        StackTrace() errors.StackTrace
}

Where errors.StackTrace is defined as 

type StackTrace []Frame

The Frame type represents a call site in the stack trace. Frame supports the fmt.Formatter interface that can be used for printing information about the stack trace of this error. For example: 

if err, ok := err.(stackTracer); ok {
        for _, f := range err.StackTrace() {
                fmt.Printf("%+s:%d", f)
        }
}

stackTracer interface is not exported by this package, but is considered a part of stable public API.

See the documentation for Frame.Format for more details.

▾ Example (StackTrace)

Code:

type stackTracer interface {
    StackTrace() errors.StackTrace
}

err, ok := errors.Cause(fn()).(stackTracer)
if !ok {
    panic("oops, err does not implement stackTracer")
}

st := err.StackTrace()
fmt.Printf("%+v", st[0:2]) // top two frames

// Example output:
// github.com/pkg/errors_test.fn
//	/home/dfc/src/github.com/pkg/errors/example_test.go:47
// github.com/pkg/errors_test.Example_stackTrace
//	/home/dfc/src/github.com/pkg/errors/example_test.go:127

func Cause 

func Cause(err error) error

Cause returns the underlying cause of the error, if possible. An error value has a cause if it implements the following interface: 

type causer interface {
       Cause() error
}

If the error does not implement Cause, the original error will be returned. If the error is nil, nil will be returned without further investigation.

▾ Example

Code:

err := fn()
fmt.Println(err)
fmt.Println(errors.Cause(err))

Output:

outer: middle: inner: error
error

▾ Example (Printf)

Code:

err := errors.Wrap(func() error {
    return func() error {
        return errors.Errorf("hello %s", fmt.Sprintf("world"))
    }()
}(), "failed")

fmt.Printf("%v", err)

Output:

failed: hello world

func Errorf 

func Errorf(format string, args ...interface{}) error

Errorf formats according to a format specifier and returns the string as a value that satisfies error. Errorf also records the stack trace at the point it was called.

▾ Example (Extended)

Code:

err := errors.Errorf("whoops: %s", "foo")
fmt.Printf("%+v", err)

// Example output:
// whoops: foo
// github.com/pkg/errors_test.ExampleErrorf
//         /home/dfc/src/github.com/pkg/errors/example_test.go:101
// testing.runExample
//         /home/dfc/go/src/testing/example.go:114
// testing.RunExamples
//         /home/dfc/go/src/testing/example.go:38
// testing.(*M).Run
//         /home/dfc/go/src/testing/testing.go:744
// main.main
//         /github.com/pkg/errors/_test/_testmain.go:102
// runtime.main
//         /home/dfc/go/src/runtime/proc.go:183
// runtime.goexit
//         /home/dfc/go/src/runtime/asm_amd64.s:2059

func New 

func New(message string) error

New returns an error with the supplied message. New also records the stack trace at the point it was called.

▾ Example

Code:

err := errors.New("whoops")
fmt.Println(err)

Output:

whoops

▾ Example (Printf)

Code:

err := errors.New("whoops")
fmt.Printf("%+v", err)

// Example output:
// whoops
// github.com/pkg/errors_test.ExampleNew_printf
//         /home/dfc/src/github.com/pkg/errors/example_test.go:17
// testing.runExample
//         /home/dfc/go/src/testing/example.go:114
// testing.RunExamples
//         /home/dfc/go/src/testing/example.go:38
// testing.(*M).Run
//         /home/dfc/go/src/testing/testing.go:744
// main.main
//         /github.com/pkg/errors/_test/_testmain.go:106
// runtime.main
//         /home/dfc/go/src/runtime/proc.go:183
// runtime.goexit
//         /home/dfc/go/src/runtime/asm_amd64.s:2059

func WithMessage 

func WithMessage(err error, message string) error

WithMessage annotates err with a new message. If err is nil, WithMessage returns nil.

▾ Example

Code:

cause := errors.New("whoops")
err := errors.WithMessage(cause, "oh noes")
fmt.Println(err)

Output:

oh noes: whoops

func WithStack 

func WithStack(err error) error

WithStack annotates err with a stack trace at the point WithStack was called. If err is nil, WithStack returns nil.

▾ Example

Code:

cause := errors.New("whoops")
err := errors.WithStack(cause)
fmt.Println(err)

Output:

whoops

▾ Example (Printf)

Code:

cause := errors.New("whoops")
err := errors.WithStack(cause)
fmt.Printf("%+v", err)

// Example Output:
// whoops
// github.com/pkg/errors_test.ExampleWithStack_printf
//         /home/fabstu/go/src/github.com/pkg/errors/example_test.go:55
// testing.runExample
//         /usr/lib/go/src/testing/example.go:114
// testing.RunExamples
//         /usr/lib/go/src/testing/example.go:38
// testing.(*M).Run
//         /usr/lib/go/src/testing/testing.go:744
// main.main
//         github.com/pkg/errors/_test/_testmain.go:106
// runtime.main
//         /usr/lib/go/src/runtime/proc.go:183
// runtime.goexit
//         /usr/lib/go/src/runtime/asm_amd64.s:2086
// github.com/pkg/errors_test.ExampleWithStack_printf
//         /home/fabstu/go/src/github.com/pkg/errors/example_test.go:56
// testing.runExample
//         /usr/lib/go/src/testing/example.go:114
// testing.RunExamples
//         /usr/lib/go/src/testing/example.go:38
// testing.(*M).Run
//         /usr/lib/go/src/testing/testing.go:744
// main.main
//         github.com/pkg/errors/_test/_testmain.go:106
// runtime.main
//         /usr/lib/go/src/runtime/proc.go:183
// runtime.goexit
//         /usr/lib/go/src/runtime/asm_amd64.s:2086

func Wrap 

func Wrap(err error, message string) error

Wrap returns an error annotating err with a stack trace at the point Wrap is called, and the supplied message. If err is nil, Wrap returns nil.

▾ Example

Code:

cause := errors.New("whoops")
err := errors.Wrap(cause, "oh noes")
fmt.Println(err)

Output:

oh noes: whoops

▾ Example (Extended)

Code:

err := fn()
fmt.Printf("%+v\n", err)

// Example output:
// error
// github.com/pkg/errors_test.fn
//         /home/dfc/src/github.com/pkg/errors/example_test.go:47
// github.com/pkg/errors_test.ExampleCause_printf
//         /home/dfc/src/github.com/pkg/errors/example_test.go:63
// testing.runExample
//         /home/dfc/go/src/testing/example.go:114
// testing.RunExamples
//         /home/dfc/go/src/testing/example.go:38
// testing.(*M).Run
//         /home/dfc/go/src/testing/testing.go:744
// main.main
//         /github.com/pkg/errors/_test/_testmain.go:104
// runtime.main
//         /home/dfc/go/src/runtime/proc.go:183
// runtime.goexit
//         /home/dfc/go/src/runtime/asm_amd64.s:2059
// github.com/pkg/errors_test.fn
// 	  /home/dfc/src/github.com/pkg/errors/example_test.go:48: inner
// github.com/pkg/errors_test.fn
//        /home/dfc/src/github.com/pkg/errors/example_test.go:49: middle
// github.com/pkg/errors_test.fn
//      /home/dfc/src/github.com/pkg/errors/example_test.go:50: outer

func Wrapf 

func Wrapf(err error, format string, args ...interface{}) error

Wrapf returns an error annotating err with a stack trace at the point Wrapf is call, and the format specifier. If err is nil, Wrapf returns nil.

▾ Example

Code:

cause := errors.New("whoops")
err := errors.Wrapf(cause, "oh noes #%d", 2)
fmt.Println(err)

Output:

oh noes #2: whoops

type Frame

Frame represents a program counter inside a stack frame. 

type Frame uintptr

func (Frame) Format 

func (f Frame) Format(s fmt.State, verb rune)

Format formats the frame according to the fmt.Formatter interface. 

%s    source file
%d    source line
%n    function name
%v    equivalent to %s:%d

Format accepts flags that alter the printing of some verbs, as follows: 

%+s   path of source file relative to the compile time GOPATH
%+v   equivalent to %+s:%d

type StackTrace

StackTrace is stack of Frames from innermost (newest) to outermost (oldest). 

type StackTrace []Frame

func (StackTrace) Format 

func (st StackTrace) Format(s fmt.State, verb rune)