by ansel1

ansel1 / merry

Golang errors with stacktrace and context

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merry Build Status GoDoc Go Report Card

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

The package is largely based on, with additional inspiration from and


go get


Merry errors work a lot like google's 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.

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


  • Support for go 2's errors.Is and errors.As functions
  • 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
  • Add a stacktrace to a sentinel error

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

    func Parse() error { // ... return ParseError.Here() // captures a stacktrace here }

  • The golang idiom for testing errors against sentinel values or type checking them doesn't work with merry errors, since they are wrapped. Use Is() for sentinel value checks, or the new go 2 errors.As() function for testing error types.

    err := Parse()

    // sentinel value check if merry.Is(err, ParseError) { // ... }

    // type check if serr, ok := merry.Unwrap(err).(*SyntaxError); ok { // ... }

    // these only work in go1.13

    // sentinel value check if errors.Is(err, ParseError) {}

    // type check var serr *SyntaxError if errors.As(err, &serr) {}

  • Add to the message on an error.

    err := merry.Prepend(ParseError, "reading config").Append("bad input")
    fmt.Println(err.Error()) // reading config: parse error: bad input
  • 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

instances. Functions with add context (e.g.
) work on any
, and will automatically convert them to merry errors (with a stack) if necessary.

Capturing the stack can be globally disabled with


Functions which get context values from errors also accept

, 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

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.


package main

import ( "" "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

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.



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

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