errors

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README

errors

Package errors provides simple error handling primitives and behavioral errors.

go get github.com/corestoreio/errors

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.

Additional inspiration:

Licence

BSD-2-Clause

Copyright (c) 2015, Dave Cheney <[email protected]>
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright notice,
  this list of conditions and the following disclaimer in the documentation
  and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Licensing

CoreStore is licensed under the Apache License, Version 2.0. See LICENSE for the full license text.

Copyright 2015-2017, Cyrill @ Schumacher.fm and the CoreStore contributors

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

Documentation

Overview

Package errors provides simple error handling primitives and behavioral errors.

Hello there! Read the presentation http://dave.cheney.net/paste/gocon-spring-2016.pdf to see what the big deal is.

http://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully

Read this for asserting errors for their behaviour http://dave.cheney.net/2014/12/24/inspecting-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 by recording a stack trace at the point Wrap is called, and 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 of annotating an error with a stack trace and an 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 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.

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, 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
}

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 err, ok := err.(stackTracer); ok {
        for _, f := range err.StackTrace() {
                fmt.Printf("%+s:%d", f)
        }
}

stackTracer interface is not exported by this package, but is considered a part of stable public API.

See the documentation for Frame.Format for more details.

Example (StackTrace) 
package main

import (
	"fmt"

	"github.com/corestoreio/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 Attach 

func Attach(err error, k Kind) error

Attach adds Kind `k` to an error but only if the error has been created with this package. For example you can create a Restricted error and then attach a Temporary kind. Now the error has two kinds. If the error hasn't been created by this package, then nothing happens.

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/corestoreio/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/corestoreio/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 CausedBehaviour 

func CausedBehaviour(err error, k Kind) bool

CausedBehaviour returns the first underlying caused kind/behaviour 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 or is nil, false will be returned. The variable `k` gets called on each unwrapped "cause" error.

func Detach 

func Detach(err error, k Kind) error

Detach opposite of Attach.

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/corestoreio/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 FormatLineFunc 

func FormatLineFunc(errs []error) string

FormatLineFunc is a basic formatter that outputs the errors that occurred along with the filename and the line number of the errors. Only if the error has a Location() function.

func Is 

func Is(err error, k Kind) bool

Is returns true if `err` is of Kind `k`. It unwraps all underlying errors which implement the Causer interface.

func MarshalAppend 

func MarshalAppend(err error, b []byte) []byte

MarshalAppend marshals an arbitrary error into a byte slice. The result is appended to b, which may be nil. It returns the argument slice unchanged if the error is nil. If the error is not an *Error, it just records the result of err.Error(). Otherwise it encodes the full Error struct.

func MatchAll 

func MatchAll(err error, k ...Kind) bool

MatchAll checks if all Kinds are included in `err`. It does not unwrap `err` by its `Causer` interface.

func MatchAny 

func MatchAny(err error, k ...Kind) bool

MatchAny checks if at least one Kind is included in `err`. It does not unwrap `err` by its `Causer` interface.

func MultiErrMatchAll 

func MultiErrMatchAll(err error, ks ...Kind) bool

MultiErrMatchAll checks if err contains a behavioral error. 1st argument err must be of type (*MultiErr) and validate function vf at least one of the many Is*() e.g. IsNotValid(), see type BehaviourFunc. All validate functions must return true. If there are multiple behavioral errors and one BehaviourFunc it will stop after all errors matches the BehaviourFunc, not at the first match.

func MultiErrMatchAny 

func MultiErrMatchAny(err error, ks ...Kind) bool

MultiErrMatchAny checks if err contains at least one behavioral error. 1st argument err must be of type (*MultiErr) and validate function vf at least one of the many Is*() e.g. IsNotValid(), see type BehaviourFunc.

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/corestoreio/errors"
)

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

}

Output:

whoops
Example (Printf) 
package main

import (
	"fmt"

	"github.com/corestoreio/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 Unmarshal 

func Unmarshal(b []byte) error

Unmarshal unmarshals the byte slice into an error value. If the slice is nil or empty, it returns nil. Otherwise the byte slice must have been created by `Marshal` or `MarshalAppend`. If the encoded error was of type `errors` within this package, the returned error value will have that underlying type. Otherwise it will be just a simple value that implements the error interface.

func UnwrapStack 

func UnwrapStack(err error) []byte

UnwrapStack tries to extract the previous stack trace after unmarshalling a byte slice into an error. It can return 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/corestoreio/errors"
)

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

}

Output:

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.

Example 
package main

import (
	"fmt"

	"github.com/corestoreio/errors"
)

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

}

Output:

whoops
Example (Printf) 
package main

import (
	"fmt"

	"github.com/corestoreio/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.

Example 
package main

import (
	"fmt"

	"github.com/corestoreio/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/corestoreio/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.

Example 
package main

import (
	"fmt"

	"github.com/corestoreio/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 ErrorFormatFunc 

type ErrorFormatFunc func([]error) string

ErrorFormatFunc is a function callback that is called by Error to turn the list of errors into a string.

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 Kind 

type Kind uint64

Kind defines the kind or behaviour of an error. An error can have multiple Kinds wrapped into each other via bit operations. A zero Kind represents an empty Kind. The underlying type uint might change, so please use the functions provided in this package to manipulate the Kind. 63 different constants are currently supported. 

const (
	Aborted Kind = 1 << iota
	AlreadyCaptured
	AlreadyClosed
	AlreadyExists
	AlreadyInUse
	AlreadyRefunded
	Blocked
	ReadFailed
	WriteFailed
	VerificationFailed
	DecryptionFailed
	EncryptionFailed
	ConnectionFailed
	BadEncoding
	ConnectionLost
	Declined
	Denied
	Duplicated
	NotEmpty
	Empty
	Exceeded
	Exists
	NotExists
	Expired
	Fatal
	InProgress
	Insufficient
	Interrupted
	IsDirectory
	IsFile
	NotDirectory
	NotFile
	Locked
	Mismatch
	NotAcceptable
	NotAllowed
	NotFound
	NotImplemented
	NotRecoverable
	NotSupported
	NotValid
	Overflowed
	PermissionDenied
	Unauthorized
	UserNotFound
	QuotaExceeded
	Rejected
	Required
	Restricted
	Revoked
	Temporary
	Terminated
	Timeout
	TooLarge
	Unavailable
	WrongVersion
	CorruptData
	OutofRange
)

These constants define different behaviours. They are not sorted and new constants must be appended at the end. The zero kind defines empty. 

const NoKind Kind = 0

NoKind defines an empty Kind with no behaviour. This constant must be placed outside the constant block to avoid a conflict with iota.

func UnwrapKind 

func UnwrapKind(err error) (k Kind)

UnwrapKind extract the Kind of an error. If the error has not been created via this package or does not implement interface `Kinder`, this function will return zero.

func (Kind) Empty 

func (k Kind) Empty() bool

Empty returns true if no behaviour/kind has been set.

func (Kind) Match 

func (k Kind) Match(err error) bool

Match returns true if `err` matches the Kind.

func (Kind) New 

func (k Kind) New(err error, msg string, args ...interface{}) error

New wraps err with the specified Kind. Allows to write an additional message which gets formatted by fmt.Sprintf.

func (Kind) Newf 

func (k Kind) Newf(format string, args ...interface{}) error

Newf creates a new error with a message formatted by fmt.Sprintf.

func (Kind) String 

func (k Kind) String() string

func (Kind) Unwrap 

func (k Kind) Unwrap() Kinds

Unwrap returns all Kind where the bit flag is set. If Kind is empty, returns nil.

type Kinder 

type Kinder interface {
	ErrorKind() Kind
}

Kinder may be implemented by any other custom error type to define desired kinds/behaviours. Multiple Kinds can be returned.

type Kinds 

type Kinds []Kind

Kinds a slice of Kind. Each Kind does not contain any other Kind.

func UnwrapKinds 

func UnwrapKinds(err error) Kinds

UnwrapKinds checks if error has a Kind/behaviour and returns a slice with all found Kinds. The returned slice is sorted numerical.

func (Kinds) String 

func (ks Kinds) String() string

String creates a comma separated list of the Kind names.

type MultiErr 

type MultiErr struct {
	Errors    []error
	Formatter ErrorFormatFunc
}

MultiErr represents a container for collecting and printing multiple errors.

func NewMultiErr 

func NewMultiErr(errs ...error) *MultiErr

NewMultiErr creates a new multi error struct.

func (*MultiErr) AppendErrors 

func (m *MultiErr) AppendErrors(errs ...error) *MultiErr

AppendErrors adds multiple errors to the container. Does not add a location. If *MultiErr is nil it creates a new pointer and returns it.

func (*MultiErr) Error 

func (m *MultiErr) Error() string

Error returns a string where each error has been separated by a line break. The location will be added to the output to show you the file name and line number.

func (*MultiErr) HasErrors 

func (m *MultiErr) HasErrors() bool

HasErrors checks if Multi contains errors.

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.

Source Files

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