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Package errors provides simple error handling primitives.

go get github.com/pkg/errors

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. For example

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

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 can be inspected by errors.Cause.

type causer interface {
        Cause() error
}

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
}

Read the package documentation for more information.

Contributing

We welcome pull requests, bug fixes and issue reports. With that said, the bar for adding new symbols to this package is intentionally set high.

Before proposing a change, please discuss your change by raising an issue.

License

BSD-2-Clause

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Documentation

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

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

If additional control is required the errors.AddStack and errors.WithMessage functions destructure errors.Annotate 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.Annotate 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.Annotate 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. errors.Unwrap is also available: this will retrieve the next error in the chain.

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, Annotate, and Annotatef record a stack trace at the point they are invoked. This information can be retrieved with the StackTracer interface that returns a 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 stacked := errors.GetStackTracer(err); stacked != nil {
        for _, f := range stacked.StackTrace() {
                fmt.Printf("%+s:%d", f)
        }
}

See the documentation for Frame.Format for more details.

errors.Find can be used to search for an error in the error chain.

Example (StackTrace)

Code:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

func fn() error {
	e1 := errors.New("error")
	e2 := errors.Wrap(e1, "inner")
	e3 := errors.Wrap(e2, "middle")
	return errors.Wrap(e3, "outer")
}

func main() {
	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
}

Index

Examples

Constants

This section is empty.

Variables

This section is empty.

Functions

func AddStack

func AddStack(err error) error

AddStack is similar to WithStack. However, it will first check with HasStack to see if a stack trace already exists in the causer chain before creating another one.

func AlreadyExistsf

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

AlreadyExistsf represents an error with already exists message.

func Annotate

func Annotate(err error, message string) error

Annotate adds a message and ensures there is a stack trace.

func Annotatef

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

Annotatef adds a message and ensures there is a stack trace.

func BadRequestf

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

BadRequestf represents an error with bad request message.

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:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

func fn() error {
	e1 := errors.New("error")
	e2 := errors.Wrap(e1, "inner")
	e3 := errors.Wrap(e2, "middle")
	return errors.Wrap(e3, "outer")
}

func main() {
	err := fn()
	fmt.Println(err)
	fmt.Println(errors.Cause(err))

}
outer: middle: inner: error
error
Example (Printf)

Code:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

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

	fmt.Printf("%v", err)

}
failed: hello world

func ErrorStack

func ErrorStack(err error) string

ErrorStack will format a stack trace if it is available, otherwise it will be Error() If the error is nil, the empty string is returned Note that this just calls fmt.Sprintf("%+v", err)

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:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

func main() {
	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 Errors

func Errors(err error) []error

Errors uses the ErrorGroup interface to return a slice of errors. If the ErrorGroup interface is not implemented it returns an array containing just the given error.

func Find

func Find(origErr error, test func(error) bool) error

Find an error in the chain that matches a test function. returns nil if no error is found.

func HasStack

func HasStack(err error) bool

HasStack tells whether a StackTracer exists in the error chain

func IsAlreadyExists

func IsAlreadyExists(err error) bool

IsAlreadyExists reports whether err was already exists error.

func IsNotFound

func IsNotFound(err error) bool

IsNotFound reports whether err was not found error.

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:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

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

}
whoops
Example (Printf)

Code:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

func main() {
	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 NewNoStackError

func NewNoStackError(msg string) error

NewNoStackError creates error without error stack later duplicate trace will no longer generate Stack too.

func NotFoundf

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

NotFoundf represents an error with not found message.

func NotSupportedf

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

NotSupportedf represents an error with not supported message.

func NotValidf

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

NotValidf represents an error with not valid message.

func SuspendStack

func SuspendStack(err error) error

SuspendStack suspends stack generate for error.

func Trace

func Trace(err error) error

Trace just calls AddStack.

func Unwrap

func Unwrap(err error) error

Unwrap uses causer to return the next error in the chain or nil. This goes one-level deeper, whereas Cause goes as far as possible

func WalkDeep

func WalkDeep(err error, visitor func(err error) bool) bool

WalkDeep does a depth-first traversal of all errors. Any ErrorGroup is traversed (after going deep). The visitor function can return true to end the traversal early In that case, WalkDeep will return true, otherwise false.

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:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

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

}
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.

For most use cases this is deprecated and AddStack should be used (which will ensure just one stack trace). However, one may want to use this in some situations, for example to create a 2nd trace across a goroutine.

Example

Code:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

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

}
whoops
Example (Printf)

Code:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

func main() {
	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.

For most use cases this is deprecated in favor of Annotate. Annotate avoids creating duplicate stack traces.

Example

Code:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

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

}
oh noes: whoops
Example (Extended)

Code:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

func fn() error {
	e1 := errors.New("error")
	e2 := errors.Wrap(e1, "inner")
	e3 := errors.Wrap(e2, "middle")
	return errors.Wrap(e3, "outer")
}

func main() {
	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.

For most use cases this is deprecated in favor of Annotatef. Annotatef avoids creating duplicate stack traces.

Example

Code:

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

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

}
oh noes #2: whoops

Types

type ErrorGroup

type ErrorGroup interface {
	Errors() []error
}

ErrorGroup is an interface for multiple errors that are not a chain. This happens for example when executing multiple operations in parallel.

type Frame

type Frame uintptr

Frame represents a program counter inside a stack frame.

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   function name and path of source file relative to the compile time
      GOPATH separated by \n\t (<funcname>\n\t<path>)
%+v   equivalent to %+s:%d

type StackTrace

type StackTrace []Frame

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

func (StackTrace) Format

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

Format formats the stack of Frames according to the fmt.Formatter interface.

%s	lists source files for each Frame in the stack
%v	lists the source file and line number for each Frame in the stack

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

%+v   Prints filename, function, and line number for each Frame in the stack.

type StackTraceAware

type StackTraceAware interface {
	HasStack() bool
}

StackTraceAware is an optimization to avoid repetitive traversals of an error chain. HasStack checks for this marker first. Annotate/Wrap and Annotatef/Wrapf will produce this marker.

type StackTracer

type StackTracer interface {
	StackTrace() StackTrace
}

StackTracer retrieves the StackTrace Generally you would want to use the GetStackTracer function to do that.

func GetStackTracer

func GetStackTracer(origErr error) StackTracer

GetStackTracer will return the first StackTracer in the causer chain. This function is used by AddStack to avoid creating redundant stack traces.

You can also use the StackTracer interface on the returned error to get the stack trace.

func NewStack

func NewStack(skip int) StackTracer

NewStack is for library implementers that want to generate a stack trace. Normally you should insted use AddStack to get an error with a stack trace.

The result of this function can be turned into a stack trace by calling .StackTrace()

This function takes an argument for the number of stack frames to skip. This avoids putting stack generation function calls like this one in the stack trace. A value of 0 will give you the line that called NewStack(0) A library author wrapping this in their own function will want to use a value of at least 1.