README
¶
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 Stacktrace interface {
Stacktrace() []Frame
}
The Frame type represents a call site in the stacktrace.
Frame supports the fmt.Formatter interface that can be used for printing information about the stacktrace of this error. For example
if err, ok := err.(Stacktrace); ok {
fmt.Printf("%+s:%d", err.Stacktrace())
}
See the documentation for Frame.Format for more details.
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
}
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 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 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
}
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 Stacktrace interface {
Stacktrace() []Frame
}
Index ¶
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
func Cause ¶
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
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
deprecated
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 one of the following interfaces:
type Stacktrace interface {
Stacktrace() []Frame
}
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.
Deprecated: Fprint will be removed in version 0.7.
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 ¶
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 Wrap ¶
Wrap returns an error annotating err with message. If err 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
Wrapf returns an error annotating err with the format specifier. If err 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 ¶
type Frame ¶ added in v0.6.0
type Frame uintptr
Frame represents an activation record.
func (Frame) Format ¶ added in v0.6.0
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
Source Files