merry

package module
v1.2.0 Latest Latest
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 Go to latest Published: Aug 24, 2018 License: MIT Imports: 5 Imported by: 273

README

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Make your golang errors merry, with stacktraces, inheritance, and arbitrary additional context.

The package is largely based on http://github.com/go-errors/errors, with additional inspiration from https://github.com/go-errgo/errgo and https://github.com/amattn/deeperror.

Installation

go get github.com/ansel1/merry

Features

Merry errors work a lot like google's golang.org/x/net/context package. Merry errors wrap normal errors with a context of key/value pairs. Like contexts, merry errors are immutable: adding a key/value to an error always creates a new error which wraps the original.

merry comes with built-in support for adding information to errors:

  • stacktraces
  • overriding the error message
  • HTTP status codes
  • End user error messages

You can also add your own additional information.

The stack capturing feature can be turned off for better performance, though it's pretty fast. Benchmarks on an 2017 MacBook Pro, with go 1.10:

BenchmarkNew_withStackCapture-8      	 2000000	       749 ns/op
BenchmarkNew_withoutStackCapture-8   	20000000	        64.1 ns/op

Details

  • New errors have a stacktrace captured where they are created

  • Add a stacktrace to existing errors (captured where they are wrapped)

    err := lib.Read()
return merry.Wrap(err)  // no-op if err is already merry

  • Allow golang idiom of comparing an err value to an exported value, using Is()

    var ParseError = merry.New("Parse error")

func Parse() error {
    err := ParseError.Here() // captures a stacktrace here
    merry.Is(err, ParseError)  // instead of err == ParseError
}

  • Change the message on an error, while still using Is() to compare to the original error

    err := merry.WithMessage(ParseError, "Bad input")
merry.Is(err, ParseError) // yes it is

  • Is() supports hierarchies of errors

    var ParseError = merry.New("Parse error")
var InvalidCharSet = merry.WithMessage(ParseError, "Invalid char set")
var InvalidSyntax = merry.WithMessage(ParseError, "Invalid syntax")

func Parse(s string) error {
    // use chainable methods to add context
    return InvalidCharSet.Here().WithMessagef("Invalid char set: %s", "UTF-8")
    // or functions
    // return merry.WithMessagef(merry.Here(InvalidCharSet), "Invalid char set: %s", "UTF-8")
}

func Check() {
    err := Parse("fields")
    merry.Is(err, ParseError) // yup
    merry.Is(err, InvalidCharSet) // yup
    merry.Is(err, InvalidSyntax) // nope
}

  • Add an HTTP status code

    merry.HTTPCode(errors.New("regular error")) // 500
merry.HTTPCode(merry.New("merry error").WithHTTPCode(404)) // 404

  • Set an alternate error message for end users

    e := merry.New("crash").WithUserMessage("nothing to see here")
merry.UserMessage(e)  // returns "nothing to see here"

  • Functions for printing error details

    err := merry.New("boom")
m := merry.Stacktrace(err) // just the stacktrace
m = merry.Details(err) // error message and stacktrace
fmt.Sprintf("%+v", err) == merry.Details(err) // errors implement fmt.Formatter

  • Add your own context info

    err := merry.New("boom").WithValue("explosive", "black powder")

Basic Usage

The package contains functions for creating new errors with stacks, or adding a stack to error instances. Functions with add context (e.g. WithValue()) work on any error, and will automatically convert them to merry errors (with a stack) if necessary.

Capturing the stack can be globally disabled with SetStackCaptureEnabled(false)

Functions which get context values from errors also accept error, and will return default values if the error is not merry, or doesn't have that key attached.

All the functions which create or attach context return concrete instances of *Error. *Error implements methods to add context to the error (they mirror the functions and do the same thing). They allow for a chainable syntax for adding context.

Example:

package main

import (
    "github.com/ansel1/merry"
    "errors"
)

var InvalidInputs = errors.New("Input is invalid")

func main() {
    // create a new error, with a stacktrace attached
    err := merry.New("bad stuff happened")
    
    // create a new error with format string, like fmt.Errorf
    err = merry.Errorf("bad input: %v", os.Args)
    
    // capture a fresh stacktrace from this callsite
    err = merry.Here(InvalidInputs)
    
    // Make err merry if it wasn't already.  The stacktrace will be captured here if the
    // error didn't already have one.  Also useful to cast to *Error 
    err = merry.Wrap(err, 0)

    // override the original error's message
    err.WithMessagef("Input is invalid: %v", os.Args)
    
    // Use Is to compare errors against values, which is a common golang idiom
    merry.Is(err, InvalidInputs) // will be true
    
    // associated an http code
    err.WithHTTPCode(400)
    
    perr := parser.Parse("blah")
    err = Wrap(perr, 0)
    // Get the original error back
    merry.Unwrap(err) == perr  // will be true
    
    // Print the error to a string, with the stacktrace, if it has one
    s := merry.Details(err)
    
    // Just print the stacktrace (empty string if err is not a RichError)
    s := merry.Stacktrace(err)

    // Get the location of the error (the first line in the stacktrace)
    file, line := merry.Location(err)
    
    // Get an HTTP status code for an error.  Defaults to 500 for non-nil errors, and 200 if err is nil.
    code := merry.HTTPCode(err)
    
}

See inline docs for more details.

Plugs

Check out my HTTP client library, github.com/gemalto/requester!

License

This package is licensed under the MIT license, see LICENSE.MIT for details.

Documentation

Overview

Package merry provides enriched golang errors, with stacktraces

merry creates errors with stacktraces, and can augment those errors with additional information.

When you create a new merry error, or wrap an existing error in a merry error, merry attaches a stacktrace to the error: 

err := merry.New("an error occurred")

err has a stacktrace attached. Alternately, you can wrap existing errors. merry will attach a stacktrace at the point of wrapping: 

_, err := ioutil.ReadAll(r)
if err != nil {
    return merry.Wrap(err)
}

Capturing the stack can be globally disabled with `SetStackCaptureEnabled(false)`. Wrapping is idempotent: Wrap will only attach a stacktrace if the error doesn't already have one.

Wrap() is the simplest way to attach a stacktrace to an error, but other functions can be used instead, with both add a stacktrace, and augment or modify the error. For example, Prepend() modifies the error's message (and also attaches a stacktrace): 

_, err := ioutil.ReadAll(r)
if err != nil {
    return merry.Prepend(err, "reading from conn failed")
    // err.Error() would read something like "reading from conn failed: timeout"
}

See the other package functions for other ways to augment or modify errors, such as Append, WithUserMessage, WithHTTPCode, WithValue, etc. These functions all return a merry.Error interface, which has methods which mirror the package level functions, to allow simple chaining: 

return merry.New("object not found").WithHTTPCode(404)

Here

Wrap will not take a new stacktrace if an error already has one attached. Here will create a new error which replaces the stacktrace with a new one: 

var ErrOverflow = merry.New("overflowed")

func Read() error {
    // ...
    return merry.Here(ErrOverflow)
}

Is

The go idiom of exporting package-level error variables for comparison to errors returned by the package is broken by merry. For example: 

_, err := io.ReadAll(r)
if err == io.EOF {
    // ...
}

If the error returned was a merry error, the equality comparison would always fail, because merry augments errors by wrapping them in layers. To compensate for this, merry has the Is() function. 

if merry.Is(err, io.EOF) {

Is() will unwrap the err and compare each layer to the second argument.

Cause

You can add a cause to an error: 

if err == io.EOF {
    err = merry.New("reading failed"), err)
    fmt.Println(err.Error()) // reading failed: EOF
}

Cause(error) will return the cause of the argument. RootCause(error) returns the innermost cause. Is(err1, err2) is cause aware, and will return true if err2 is a cause (anywhere in the causal change) of err1.

Formatting and printing

To obtain an error's stacktrace, call Stack(). To get other information about the site of the error, or print the error's stacktrace, see Location(), SourceLine(), Stacktrace(), and Details().

merry errors also implement the fmt.Formatter interface. errors support the following fmt flags: 

%+v   print the equivalent of Details(err), which includes the user message, full stacktrace,
      and recursively prints the details of the cause chain.

Index

Constants

This section is empty.

Variables

View Source 
var MaxStackDepth = 50

MaxStackDepth is the maximum number of stackframes on any error.

Functions

func Cause added in v1.2.0 

func Cause(e error) error

Cause returns the cause of the argument. If e is nil, or has no cause, nil is returned.

func Details 

func Details(e error) string

Details returns e.Error() and e's stacktrace and user message, if set.

func HTTPCode 

func HTTPCode(e error) int

HTTPCode converts an error to an http status code. All errors map to 500, unless the error has an http code attached. If e is nil, returns 200.

func Is 

func Is(e error, originals ...error) bool

Is checks whether e is equal to or wraps the original, at any depth. If e == nil, return false. This is useful if your package uses the common golang pattern of exported error constants. If your package exports an ErrEOF constant, which is initialized like this: 

var ErrEOF = errors.New("End of file error")

...and your user wants to compare an error returned by your package with ErrEOF: 

err := urpack.Read()
if err == urpack.ErrEOF {

...the comparison will fail if the error has been wrapped by merry at some point. Replace the comparison with: 

if merry.Is(err, urpack.ErrEOF) {

Causes

Is will also return true if any of the originals is in the cause chain of e. For example: 

e1 := merry.New("base error")
e2 := merry.New("library error")
// e2 was caused by e1
e3 := merry.WithCause(e1, e2)
merry.Is(e3, e2)  // yes it is, because e3 is based on e2
merry.Is(e3, e1)  // yes it is, because e1 was a cause of e3

func Location 

func Location(e error) (file string, line int)

Location returns zero values if e has no stacktrace

func Message 

func Message(e error) string

Message returns just the error message. It is equivalent to Error() when Verbose is false. The behavior of Error() is (pseudo-code): 

if verbose
  Details(e)
else
  Message(e) || UserMessage(e)

If e is nil, returns "".

func RootCause added in v1.2.0 

func RootCause(e error) error

RootCause returns the innermost cause of the argument (i.e. the last error in the cause chain)

func SetStackCaptureEnabled 

func SetStackCaptureEnabled(enabled bool)

SetStackCaptureEnabled sets stack capturing globally. Disabling stack capture can increase performance

func SetVerboseDefault 

func SetVerboseDefault(b bool)

SetVerboseDefault sets the global default for verbose mode. When true, e.Error() == Details(e) When false, e.Error() == Message(e)

func SourceLine 

func SourceLine(e error) string

SourceLine returns the string representation of Location's result or an empty string if there's no stracktrace.

func Stack 

func Stack(e error) []uintptr

Stack returns the stack attached to an error, or nil if one is not attached If e is nil, returns nil.

func StackCaptureEnabled 

func StackCaptureEnabled() bool

StackCaptureEnabled returns whether stack capturing is enabled

func Stacktrace 

func Stacktrace(e error) string

Stacktrace returns the error's stacktrace as a string formatted the same way as golangs runtime package. If e has no stacktrace, returns an empty string.

func Unwrap 

func Unwrap(e error) error

Unwrap returns the innermost underlying error. Only useful in advanced cases, like if you need to cast the underlying error to some type to get additional information from it. If e == nil, return nil.

func UserMessage 

func UserMessage(e error) string

UserMessage returns the end-user safe message. Returns empty if not set. If e is nil, returns "".

func Value 

func Value(e error, key interface{}) interface{}

Value returns the value for key, or nil if not set. If e is nil, returns nil.

func Values 

func Values(e error) map[interface{}]interface{}

Values returns a map of all values attached to the error If a key has been attached multiple times, the map will contain the last value mapped If e is nil, returns nil.

func VerboseDefault 

func VerboseDefault() bool

VerboseDefault returns the global default for verbose mode. When true, e.Error() == Details(e) When false, e.Error() == Message(e)

func WithCause added in v1.2.0 

func WithCause(e error, cause error) error

WithCause returns an error based on the first argument, with the cause set to the second argument. If e is nil, returns nil.

Types

type Error 

type Error interface {
	error
	Appendf(format string, args ...interface{}) Error
	Append(msg string) Error
	Prepend(msg string) Error
	Prependf(format string, args ...interface{}) Error
	WithMessage(msg string) Error
	WithMessagef(format string, args ...interface{}) Error
	WithUserMessage(msg string) Error
	WithUserMessagef(format string, args ...interface{}) Error
	WithValue(key, value interface{}) Error
	Here() Error
	WithStackSkipping(skip int) Error
	WithHTTPCode(code int) Error
	WithCause(err error) Error
	Cause() error
	fmt.Formatter
}

Error extends the standard golang `error` interface with functions for attachment additional data to the error

func Append 

func Append(e error, msg string) Error

Append a message after the current error message, in the format "original: new". If e == nil, return nil.

func Appendf 

func Appendf(e error, format string, args ...interface{}) Error

Appendf is the same as Append, but uses fmt.Sprintf().

func Errorf 

func Errorf(format string, a ...interface{}) Error

Errorf creates a new error with a formatted message and a stack. The equivalent of golang's fmt.Errorf()

func Here 

func Here(e error) Error

Here returns an error with a new stacktrace, at the call site of Here(). Useful when returning copies of exported package errors. If e is nil, returns nil.

func HereSkipping added in v1.1.0 

func HereSkipping(e error, skip int) Error

HereSkipping returns an error with a new stacktrace, at the call site of HereSkipping() - skip frames.

func New 

func New(msg string) Error

New creates a new error, with a stack attached. The equivalent of golang's errors.New()

func Prepend 

func Prepend(e error, msg string) Error

Prepend a message before the current error message, in the format "new: original". If e == nil, return nil.

func Prependf 

func Prependf(e error, format string, args ...interface{}) Error

Prependf is the same as Prepend, but uses fmt.Sprintf()

func UserError 

func UserError(msg string) Error

UserError creates a new error with a message intended for display to an end user.

func UserErrorf 

func UserErrorf(format string, a ...interface{}) Error

UserErrorf is like UserError, but uses fmt.Sprintf()

func WithHTTPCode 

func WithHTTPCode(e error, code int) Error

WithHTTPCode returns an error with an http code attached. If e is nil, returns nil.

func WithMessage 

func WithMessage(e error, msg string) Error

WithMessage returns an error with a new message. The resulting error's Error() method will return the new message. If e is nil, returns nil.

func WithMessagef 

func WithMessagef(e error, format string, a ...interface{}) Error

WithMessagef is the same as WithMessage(), using fmt.Sprintf().

func WithUserMessage 

func WithUserMessage(e error, msg string) Error

WithUserMessage adds a message which is suitable for end users to see. If e is nil, returns nil.

func WithUserMessagef 

func WithUserMessagef(e error, format string, args ...interface{}) Error

WithUserMessagef is the same as WithMessage(), using fmt.Sprintf()

func WithValue 

func WithValue(e error, key, value interface{}) Error

WithValue adds a context an error. If the key was already set on e, the new value will take precedence. If e is nil, returns nil.

func Wrap 

func Wrap(e error) Error

Wrap turns the argument into a merry.Error. If the argument already is a merry.Error, this is a no-op. If e == nil, return nil

func WrapSkipping 

func WrapSkipping(e error, skip int) Error

WrapSkipping turns the error arg into a merry.Error if the arg is not already a merry.Error. If e is nil, return nil. If a merry.Error is created by this call, the stack captured will skip `skip` frames (0 is the call site of `WrapSkipping()`)

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