Package eris is an error handling library with readable stack traces and flexible formatting support.

go get github.com/rotisserie/eris

• Why you should switch to eris
• Using eris
  • Creating errors
  • Wrapping errors
  • Formatting and logging errors
  • Interpreting eris stack traces
  • Inverting the stack trace and error output
  • Inspecting errors
  • Formatting with custom separators
  • Writing a custom output format
  • Sending error traces to Sentry
• Comparison to other packages (e.g. pkg/errors)
  • Error formatting and stack traces
• Migrating to eris
• Contributing

This package was inspired by a simple question: what if you could fix a bug without wasting time replicating the issue or digging through the code? With that in mind, this package is designed to give you more control over error handling via error wrapping, stack tracing, and output formatting.

The example that generated the output below simulates a realistic error handling scenario and demonstrates how to wrap and log errors with minimal effort. This specific error occurred because a user tried to access a file that can't be located, and the output shows a clear path from the top of the call stack to the source.

{
  "error":{
    "root":{
      "message":"error internal server",
      "stack":[
        "main.main:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:143",
        "main.ProcessResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:85",
        "main.ProcessResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:82",
        "main.GetRelPath:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:61"
      ]
    },
    "wrap":[
      {
        "message":"failed to get relative path for resource 'res2'",
        "stack":"main.ProcessResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:85"
      },
      {
        "message":"Rel: can't make ./some/malformed/absolute/path/data.json relative to /Users/roti/",
        "stack":"main.GetRelPath:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:61"
      }
    ]
  },
  "level":"error",
  "method":"ProcessResource",
  "msg":"method completed with error",
  "time":"2020-01-16T11:20:01-05:00"
}

Many of the methods in this package will look familiar if you've used pkg/errors or xerrors, but eris employs some additional tricks during error wrapping and unwrapping that greatly improve the readability of the stack trace. This package also takes a unique approach to formatting errors that allows you to write custom formats that conform to your error or log aggregator of choice. You can find more information on the differences between eris and pkg/errors here.

Creating errors is simple via eris.New.

var (
  // global error values can be useful when wrapping errors or inspecting error types
  ErrInternalServer = eris.New("error internal server")
)

func (req *Request) Validate() error {
  if req.ID == "" {
    // or return a new error at the source if you prefer
    return eris.New("error bad request")
  }
  return nil
}

eris.Wrap adds context to an error while preserving the original error.

relPath, err := GetRelPath("/Users/roti/", resource.AbsPath)
if err != nil {
  // wrap the error if you want to add more context
  return nil, eris.Wrapf(err, "failed to get relative path for resource '%v'", resource.ID)
}

eris.ToString and eris.ToJSON should be used to log errors with the default format (shown above). The JSON method returns a map[string]interface{} type for compatibility with Go's encoding/json package and many common JSON loggers (e.g. logrus).

// format the error to JSON with the default format and stack traces enabled
formattedJSON := eris.ToJSON(err, true)
fmt.Println(json.Marshal(formattedJSON)) // marshal to JSON and print
logger.WithField("error", formattedJSON).Error() // or ideally, pass it directly to a logger

// format the error to a string and print it
formattedStr := eris.ToString(err, true)
fmt.Println(formattedStr)

eris also enables control over the default format's separators and allows advanced users to write their own custom output format.

Errors created with this package contain stack traces that are managed automatically. They're currently mandatory when creating and wrapping errors but optional when printing or logging. By default, the stack trace and all wrapped layers follow the opposite order of Go's runtime package, which means that the original calling method is shown first and the root cause of the error is shown last.

{
  "root":{
    "message":"error bad request", // root cause
    "stack":[
      "main.main:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:143", // original calling method
      "main.ProcessResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:71",
      "main.(*Request).Validate:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:29", // location of Wrap call
      "main.(*Request).Validate:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:28" // location of the root
    ]
  },
  "wrap":[
    {
      "message":"received a request with no ID", // additional context
      "stack":"main.(*Request).Validate:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:29" // location of Wrap call
    }
  ]
}

If you prefer some other order than the default, eris supports inverting both the stack trace and the entire error output. When both are inverted, the root error is shown first and the original calling method is shown last.

// create a default format with error and stack inversion options
format := eris.NewDefaultStringFormat(eris.FormatOptions{
  InvertOutput: true, // flag that inverts the error output (wrap errors shown first)
  WithTrace: true,    // flag that enables stack trace output
  InvertTrace: true,  // flag that inverts the stack trace output (top of call stack shown first)
})

// format the error to a string and print it
formattedStr := eris.ToCustomString(err, format)
fmt.Println(formattedStr)

// example output:
// error not found
//   main.GetResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:52
//   main.ProcessResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:76
//   main.main:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:143
// failed to get resource 'res1'
//   main.GetResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:52

The eris package provides a couple ways to inspect and compare error types. eris.Is returns true if a particular error appears anywhere in the error chain. Currently, it works simply by comparing error messages with each other. If an error contains a particular message (e.g. "error not found") anywhere in its chain, it's defined to be that error type.

ErrNotFound := eris.New("error not found")
_, err := db.Get(id)
// check if the resource was not found
if eris.Is(err, ErrNotFound) {
  // return the error with some useful context
  return eris.Wrapf(err, "error getting resource '%v'", id)
}

eris.As finds the first error in a chain that matches a given target. If there's a match, it sets the target to that error value and returns true.

var target *NotFoundError
_, err := db.Get(id)
// check if the error is a NotFoundError type
if errors.As(err, &target) {
    // err is a *NotFoundError and target is set to the error's value
    return target
}

eris.Cause unwraps an error until it reaches the cause, which is defined as the first (i.e. root) error in the chain.

ErrNotFound := eris.New("error not found")
_, err := db.Get(id)
// compare the cause to some sentinel value
if eris.Cause(err) == ErrNotFound {
  // return the error with some useful context
  return eris.Wrapf(err, "error getting resource '%v'", id)
}

For users who need more control over the error output, eris allows for some control over the separators between each piece of the output via the eris.Format type. If this isn't flexible enough for your needs, see the custom output format section below. To format errors with custom separators, you can define and pass a format object to eris.ToCustomString or eris.ToCustomJSON.

// format the error to a string with custom separators
formattedStr := eris.ToCustomString(err, Format{
  FormatOptions: eris.FormatOptions{
    WithTrace: true,   // flag that enables stack trace output
  },
  MsgStackSep: "\n",   // separator between error messages and stack frame data
  PreStackSep: "\t",   // separator at the beginning of each stack frame
  StackElemSep: " | ", // separator between elements of each stack frame
  ErrorSep: "\n",      // separator between each error in the chain
})
fmt.Println(formattedStr)

// example output:
// error reading file 'example.json'
//   main.readFile | .../example/main.go | 6
// unexpected EOF
//   main.main | .../example/main.go | 20
//   main.parseFile | .../example/main.go | 12
//   main.readFile | .../example/main.go | 6

eris also allows advanced users to construct custom error strings or objects in case the default error doesn't fit their requirements. The UnpackedError object provides a convenient and developer friendly way to store and access existing error traces. The ErrRoot and ErrChain fields correspond to the root error and wrap error chain, respectively. If a root error wraps an external error, that error will be default formatted and assigned to the ErrExternal field. If any other error type is unpacked, it will appear in the ErrExternal field. You can access all of the information contained in an error via eris.Unpack.

// get the unpacked error object
uErr := eris.Unpack(err)
// send only the root error message to a logging server instead of the complete error trace
sentry.CaptureMessage(uErr.ErrRoot.Msg)

eris supports sending your error traces to Sentry using the Sentry Go client SDK. You can run the example that generated the following output on Sentry UI using the command go run examples/sentry/example.go -dsn=<DSN>.

*eris.wrapError: test: wrap 1: wrap 2: wrap 3
  File "main.go", line 19, in Example
    return eris.New("test")
  File "main.go", line 23, in WrapExample
    err := Example()
  File "main.go", line 25, in WrapExample
    return eris.Wrap(err, "wrap 1")
  File "main.go", line 31, in WrapSecondExample
    err := WrapExample()
  File "main.go", line 33, in WrapSecondExample
    return eris.Wrap(err, "wrap 2")
  File "main.go", line 44, in main
    err := WrapSecondExample()
  File "main.go", line 45, in main
    err = eris.Wrap(err, "wrap 3")

Readability is a major design requirement for eris. In addition to the JSON output shown above, eris also supports formatting errors to a simple string.

failed to get resource 'res1'
  main.GetResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:52
error not found
  main.main:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:143
  main.ProcessResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:76
  main.GetResource:/Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:52

The eris error stack is designed to be easier to interpret than other error handling packages, and it achieves this by omitting extraneous information and avoiding unnecessary repetition. The stack trace above omits calls from Go's runtime package and includes just a single frame for wrapped layers which are inserted into the root error stack trace in the correct order. eris also correctly handles and updates stack traces for global error values in a transparent way.

The output of pkg/errors for the same error is shown below. In this case, the root error stack trace is incorrect because it was declared as a global value, and it includes several extraneous lines from the runtime package. The output is also much more difficult to read and does not allow for custom formatting.

error not found
main.init
  /Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:18
runtime.doInit
  /usr/local/Cellar/go/1.13.6/libexec/src/runtime/proc.go:5222
runtime.main
  /usr/local/Cellar/go/1.13.6/libexec/src/runtime/proc.go:190
runtime.goexit
  /usr/local/Cellar/go/1.13.6/libexec/src/runtime/asm_amd64.s:1357
failed to get resource 'res1'
main.GetResource
  /Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:52
main.ProcessResource
  /Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:76
main.main
  /Users/roti/go/src/github.com/rotisserie/eris/examples/logging/example.go:143
runtime.main
  /usr/local/Cellar/go/1.13.6/libexec/src/runtime/proc.go:203
runtime.goexit
  /usr/local/Cellar/go/1.13.6/libexec/src/runtime/asm_amd64.s:1357

Migrating to eris should be a very simple process. If it doesn't offer something that you currently use from existing error packages, feel free to submit an issue to us. If you don't want to refactor all of your error handling yet, eris should work relatively seamlessly with your existing error types. Please submit an issue if this isn't the case for some reason.

Many of your dependencies will likely still use pkg/errors for error handling. When external error types are wrapped with additional context, eris creates a new root error that wraps the original external error. Because of this, error inspection should work seamlessly with other error libraries.

If you'd like to contribute to eris, we'd love your input! Please submit an issue first so we can discuss your proposal.

Released under the MIT License.