errors

package module
Version: v0.3.0 Latest Latest
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Published: May 2, 2016 License: BSD-2-Clause Imports: 6 Imported by: 0

README

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Package errors implements functions for manipulating 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")
}

In addition, errors.Wrap records the file and line where it was called, allowing the programmer to retrieve the path to the original error.

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 recurse 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
}

Would you like to know more? Read the blog post.

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.

Licence

MIT

Documentation

Overview

Package errors implements functions for manipulating 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")
}

In addition, errors.Wrap records the file and line where it was called, allowing the programmer to retrieve the path to the original error.

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 nor 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

Examples

Constants

This section is empty.

Variables

This section is empty.

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.

Example
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))

}
Output:

outer: middle: inner: error
error

func Errorf added in v0.3.0

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

Errorf formats according to a format specifier and returns the string as a value that satisfies error.

Example
package main

import (
	"os"

	"github.com/pkg/errors"
)

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

}
Output:

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. The format of the output is the same as Print. If err is nil, nothing is printed.

Example
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)

}
Output:

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.

Example
package main

import (
	"fmt"

	"github.com/pkg/errors"
)

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

}
Output:

whoops
Example (Fprint)
package main

import (
	"os"

	"github.com/pkg/errors"
)

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

}
Output:

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

func Print

func Print(err error)

Print prints the error to Stderr. 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.

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.

Example
package main

import (
	"fmt"

	"github.com/pkg/errors"
)

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

}
Output:

oh noes: whoops

func Wrapf added in v0.2.0

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.

Example
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)

}
Output:

oh noes #2: whoops

Types

This section is empty.

Source Files

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