package errors

import "github.com/pkg/errors"

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. For example

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

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 Stack interface {
        Stack() []uintptr
}

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
}

Index ¶

• func Cause(err error) error
• func Errorf(format string, args ...interface{}) error
• func Fprint(w io.Writer, err error)
• func New(text string) error
• func Wrap(cause error, message string) error
• func Wrapf(cause error, format string, args ...interface{}) error

Examples ¶

• Cause
• Errorf
• Fprint
• New
• New (Fprint)
• Wrap
• Wrapf

Functions ¶

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.

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

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.

package main

import (
	"os"

	"github.com/pkg/errors"
)

func main() {
	err := errors.Errorf("whoops: %s", "foo")
	errors.Fprint(os.Stdout, err)

}

github.com/pkg/errors/example_test.go:67: whoops: foo

func Fprint ¶

func Fprint(w io.Writer, err error)

Fprint prints the error to the supplied writer. If the error implements the Causer interface described in Cause Print will recurse into the error's cause. If the error implements the inteface:

type Location interface {
       Location() (file string, line int)
}

Print will also print the file and line of the error. If err is nil, nothing is printed.

package main

import (
	"os"

	"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()
	errors.Fprint(os.Stdout, err)

}

github.com/pkg/errors/example_test.go:36: outer
github.com/pkg/errors/example_test.go:35: middle
github.com/pkg/errors/example_test.go:34: inner
github.com/pkg/errors/example_test.go:33: error

func New ¶

func New(text string) error

New returns an error that formats as the given text.

package main

import (
	"fmt"

	"github.com/pkg/errors"
)

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

}

whoops

package main

import (
	"os"

	"github.com/pkg/errors"
)

func main() {
	err := errors.New("whoops")
	errors.Fprint(os.Stdout, err)

}

github.com/pkg/errors/example_test.go:18: whoops

func Wrap ¶

func Wrap(cause error, message string) error

Wrap returns an error annotating the cause with message. If cause is nil, Wrap returns nil.

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

func Wrapf ¶

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

Wrapf returns an error annotating the cause with the format specifier. If cause is nil, Wrapf returns nil.

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

Source Files ¶

errors.go