errors

README

errors

This package is a fork from github.com/pkg/errors package created by Dave Cheney. The original package has no longer accepting proposals for new functionality.

With the new errors on go 1.13, the way to using the errors on Go has some changes that can be applied into Dave Cheney library. We want to offer one way to migrate your code to new errors, but with the minimum refactor, for that we've created this package.

This package provide the same interface that the original library have, but using new go 1.13 errors, or in previous version golang.org/x/xerrors package.

How to start using friendsofgo/errors

If you previously was using the package github.com/pkg/errors, you only need change your imports for github.com/pantsmann/errors, with this simple change now you're capable to use go 1.13 in your code, and use the new methods As and Is if you want.

Furthermore the methods Wrap and Wrapf become compatible with Unwrap interface of new go 1.13 errors.

Adding context to an error

With the original package go 1.13 if you want add context, ergo wrap your error you need to create a new error and using the new verb "%w" like that:

_, err := ioutil.ReadAll(r)
if err != nil {
        return fmt.Errorf("read failed: %w", err)
}

Using our library you can do that forgetting to the new verb:

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

Retrieving the cause of an error

We want to keep the compatibility with the github.com/pkg/errors package, for that our package provides a Cause method, but this method is not longer needed, because we can use the new methods Is or As that provides the official package.

So previously if you needed to check an error cause, your error must be implemented the causer inteface:

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
}

But now you can do:

var target *MyError
if errors.As(err, &target) {
    // handle specifically
} else {
   // unknown error
}

Or if you uses a sentinel error:

var ErrMyError = errors.New("my sentinel error")
if errors.Is(err, ErrMyError) {
    // handle specifically
} else {
   // unknown error
}

Disclaimer

This package was created to using with go 1.13 version however if you uses this package with a previous version, the methods As, Is, Wrap and Wrapf will be using golang.org/x/xerrors package.

Contributing

Contributions are more than welcome, if you are interested please fork this repo and send your Pull Request.

Documentation

Overview

Package errors provides simple error handling primitives including stack, annotations, and data.

Original package created by Dave Cheney Copyright (c) 2015, Dave Cheney <dave@cheney.net>

Modifications of the original package by Friends of Go Copyright (c) 2019, Friends of Go <contact@friendsofgo.tech>

Further modifications of the Friends of Go version by Adam Manwaring Copyright (c) 2019, Adam Manwaring <pantsmann@byu.net>

The traditional error handling idiom in Go is roughly akin to

if err != nil {
        return err
}

which when 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, together with 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: annotating an error with a stack trace and with 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 that 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
}

Although the causer interface is not exported by this package, it is considered a part of its stable public interface.

With the new standard package error we have two new ways to figure what is the cause of our error:

var target *MyError
if errors.As(err, &target) {
       // handle specifically
} else {
       // unknown error
}

or even with sentinel errors:

var ErrMyError = errors.New("my sentinel error")
if errors.Is(err, ErrMyError) {
      // handle specifically
} else {
     // unknown error
}

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 is a wrapper it will be
      printed recursively.
%v    see %s
%+v   extended format. Each Frame of the error's StackTrace will
      be printed in detail. Any key/value pairs recorded WithData()
      will also be printed.

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
}

The returned errors.StackTrace type 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\n", f, f)
        }
}

Although the stackTracer interface is not exported by this package, it is considered a part of its stable public interface.

See the documentation for Frame.Format for more details.

Retrieving data from an error chain

Key/value pairs recorded (using WithData or WrapWithData) in errors of the error chain can be retrieved with the following interface:

type dataCacher interface {
     DataCache() map[string]interface{}
}

The returned map will contain all the key/value pairs from any dataCachers of equal or greater depth in the error chain. For example (assuming the dataError definition above):

var d dataError
if errors.As(err, &d) {
     kv := d.DataCache()
}

Note that if more than one error in the chain contains the same key, DataCache() returns only the shallowest value for that key. That is:

err = errors.WithData(err, "deep", 1, "dup", 1)
kv := errors.WithData(err, "dup" 2, "shallow", 2).DataCache()
fmt.Printf("key/value pairs: %v", kv)

// Example Output:
// key/value pairs: map[deep:1 dup:2 shallow:2]

The when using %+v to format the error all keys and values are output at the level they were set.

Example (StackTrace)

package main

import (
	"fmt"

	"github.com/pantsmann/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/pantsmann/errors_test.fn
	//	/home/dfc/src/github.com/pantsmann/errors/example_test.go:47
	// github.com/pantsmann/errors_test.Example_stackTrace
	//	/home/dfc/src/github.com/pantsmann/errors/example_test.go:127
}

Index

• func As(err error, target interface{}) bool
• func Cause(err error) error
• func Errorf(format string, args ...interface{}) error
• func Is(err error, target error) bool
• func New(message string) error
• func Unwrap(err error) error
• func WithData(err error, keyVals ...interface{}) error
• func WithMessage(err error, message string) error
• func WithMessagef(err error, format string, args ...interface{}) error
• func WithStack(err error) error
• func Wrap(err error, message string) error
• func WrapWithData(err error, message string, keyVals ...interface{}) error
• func Wrapf(err error, format string, args ...interface{}) error
• type Annotatable
• type Base
  • func (b Base) Error() string
  • func (b Base) Format(f fmt.State, verb rune)
  • func (b Base) Unwrap() error
• type Frame
  • func (f Frame) Format(s fmt.State, verb rune)
  • func (f Frame) MarshalText() ([]byte, error)
• type StackTrace
  • func (st StackTrace) Format(s fmt.State, verb rune)
• type Wrapper

Examples

• Package (StackTrace)
• Cause
• Cause (Printf)
• Errorf (Extended)
• New
• New (Printf)
• WithMessage
• WithStack
• WithStack (Printf)
• Wrap
• Wrap (Extended)
• Wrapf

Functions

func As

func As(err error, target interface{}) bool

As finds the first error in err's chain that matches target, and if so, sets target to that error value and returns true.

The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.

An error matches target if the error's concrete value is assignable to the value pointed to by target, or if the error has a method As(interface{}) bool such that As(target) returns true. In the latter case, the As method is responsible for setting target.

As will panic if target is not a non-nil pointer to either a type that implements error, or to any interface type. As returns false if err is nil.

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 standard errors.Wrapper interface:

type Wrapper interface {
       Unwrap() error
}

If the error does not implement Wrapper, 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/pantsmann/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

Example (Printf)

package main

import (
	"fmt"

	"github.com/pantsmann/errors"
)

func main() {
	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)

package main

import (
	"fmt"

	"github.com/pantsmann/errors"
)

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

	// Example output:
	// whoops: foo
	// github.com/pantsmann/errors_test.ExampleErrorf
	//         /home/dfc/src/github.com/pantsmann/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/pantsmann/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 Is

func Is(err error, target error) bool

Is reports whether any error in err's chain matches target.

The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.

An error is considered to match a target if it is equal to that target or if it implements a method Is(error) bool such that Is(target) returns true.

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

package main

import (
	"fmt"

	"github.com/pantsmann/errors"
)

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

}

Output:
whoops

Example (Printf)

package main

import (
	"fmt"

	"github.com/pantsmann/errors"
)

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

	// Example output:
	// whoops
	// github.com/pantsmann/errors_test.ExampleNew_printf
	//         /home/dfc/src/github.com/pantsmann/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/pantsmann/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 Unwrap

func Unwrap(err error) error

Unwrap returns the result of calling the Unwrap method on err, if err implements Unwrap. Otherwise, Unwrap returns nil.

func WithData

func WithData(err error, keyVals ...interface{}) error

WithData annotates err with a map of key/value pairs. keyVals should be passed in as pairs; the first of each pair being a string (the key). If an odd number of keyVals are passed in, the last one is ignored. If a key value is not a string, it and the next keyVal element (the value) are skipped. If err is nil, WithData returns nil.

func WithMessage

func WithMessage(err error, message string) error

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

Example

package main

import (
	"fmt"

	"github.com/pantsmann/errors"
)

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

}

Output:
oh noes: whoops

func WithMessagef

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

WithMessagef annotates err with the format specifier. If err is nil, WithMessagef returns nil.

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

package main

import (
	"fmt"

	"github.com/pantsmann/errors"
)

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

}

Output:
whoops

Example (Printf)

package main

import (
	"fmt"

	"github.com/pantsmann/errors"
)

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

	// Example Output:
	// whoops
	// github.com/pantsmann/errors_test.ExampleWithStack_printf
	//         /home/fabstu/go/src/github.com/pantsmann/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/pantsmann/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/pantsmann/errors_test.ExampleWithStack_printf
	//         /home/fabstu/go/src/github.com/pantsmann/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/pantsmann/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

package main

import (
	"fmt"

	"github.com/pantsmann/errors"
)

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

}

Output:
oh noes: whoops

Example (Extended)

package main

import (
	"fmt"

	"github.com/pantsmann/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/pantsmann/errors_test.fn
	//         /home/dfc/src/github.com/pantsmann/errors/example_test.go:47
	// github.com/pantsmann/errors_test.ExampleCause_printf
	//         /home/dfc/src/github.com/pantsmann/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/pantsmann/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/pantsmann/errors_test.fn
	// 	  /home/dfc/src/github.com/pantsmann/errors/example_test.go:48: inner
	// github.com/pantsmann/errors_test.fn
	//        /home/dfc/src/github.com/pantsmann/errors/example_test.go:49: middle
	// github.com/pantsmann/errors_test.fn
	//      /home/dfc/src/github.com/pantsmann/errors/example_test.go:50: outer
}

func WrapWithData

func WrapWithData(err error, message string, keyVals ...interface{}) error

WrapWithData returns an error annotating err with a stack trace at the point WrapWithData is called, the supplied message, and any supplied key/value pairs. If err is nil, WrapWithData returns nil.

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 called, and the format specifier. If err is nil, Wrapf returns nil.

Example

package main

import (
	"fmt"

	"github.com/pantsmann/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 Annotatable

type Annotatable interface {
	fmt.Formatter
	Wrapper
}

Annotatable represents errors that are unwrappable and formatable. Errors that implement special behaviors should implement this interface either directly or through embedding in order to avoid breaking Wrap() semantics. As a simple example, given the following definitions:

type mySpecial struct { errors.Annotatable }
func (m mySpecial) Special() bool { return true }

the following code would add the Special() behavior to a wrapped error

specialErr := mySpecial{errors.Wrap(someGenericError, "You are now a special error").(errors.Annotatable)}

type Base

type Base struct{ Err error }

Base is an error with basic annotatable functionality. It can be used to simply add Annotatable to special error behaviors by extending an embedded error to fulfill Annotatable. For example given the following definitions:

type mySpecial struct { errors.Annotatable }
func (m mySpecial) Special() bool { return true }

the following would add Special() as a behavior to someGenericError and this would be findable using errors.As and one would still be able to output stack traces inside of someGenericError (if they exist).

specialErr := mySpecial{errors.Base{someGenericError}}

func (Base) Error

func (b Base) Error() string

Error returns the error as a string

func (Base) Format

func (b Base) Format(f fmt.State, verb rune)

Format outputs the error based on the given format state and verbs

func (Base) Unwrap

func (b Base) Unwrap() error

Unwrap returns the internal error

type Frame

type Frame uintptr

Frame represents a program counter inside a stack frame. For historical reasons if Frame is interpreted as a uintptr its value represents the program counter + 1.

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

func (Frame) MarshalText

func (f Frame) MarshalText() ([]byte, error)

MarshalText formats a stacktrace Frame as a text string. The output is the same as that of fmt.Sprintf("%+v", f), but without newlines or tabs.

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 Wrapper

type Wrapper interface {
	error
	Unwrap() error
}

A Wrapper provides context around another error.