# go-multierror
[][travis]
[][godocs]
[travis]: https://travis-ci.org/hashicorp/go-multierror
[godocs]: https://godoc.org/github.com/hashicorp/go-multierror
`go-multierror` is a package for Go that provides a mechanism for
representing a list of `error` values as a single `error`.
This allows a function in Go to return an `error` that might actually
be a list of errors. If the caller knows this, they can unwrap the
list and access the errors. If the caller doesn't know, the error
formats to a nice human-readable format.
`go-multierror` is fully compatible with the Go standard library
[errors](https://golang.org/pkg/errors/) package, including the
functions `As`, `Is`, and `Unwrap`. This provides a standardized approach
for introspecting on error values.
## Installation and Docs
Install using `go get github.com/hashicorp/go-multierror`.
Full documentation is available at
http://godoc.org/github.com/hashicorp/go-multierror
## Usage
go-multierror is easy to use and purposely built to be unobtrusive in
existing Go applications/libraries that may not be aware of it.
**Building a list of errors**
The `Append` function is used to create a list of errors. This function
behaves a lot like the Go built-in `append` function: it doesn't matter
if the first argument is nil, a `multierror.Error`, or any other `error`,
the function behaves as you would expect.
```go
var result error
if err := step1(); err != nil {
result = multierror.Append(result, err)
}
if err := step2(); err != nil {
result = multierror.Append(result, err)
}
return result
```
**Customizing the formatting of the errors**
By specifying a custom `ErrorFormat`, you can customize the format
of the `Error() string` function:
```go
var result *multierror.Error
// ... accumulate errors here, maybe using Append
if result != nil {
result.ErrorFormat = func([]error) string {
return "errors!"
}
}
```
**Accessing the list of errors**
`multierror.Error` implements `error` so if the caller doesn't know about
multierror, it will work just fine. But if you're aware a multierror might
be returned, you can use type switches to access the list of errors:
```go
if err := something(); err != nil {
if merr, ok := err.(*multierror.Error); ok {
// Use merr.Errors
}
}
```
You can also use the standard [`errors.Unwrap`](https://golang.org/pkg/errors/#Unwrap)
function. This will continue to unwrap into subsequent errors until none exist.
**Extracting an error**
The standard library [`errors.As`](https://golang.org/pkg/errors/#As)
function can be used directly with a multierror to extract a specific error:
```go
// Assume err is a multierror value
err := somefunc()
// We want to know if "err" has a "RichErrorType" in it and extract it.
var errRich RichErrorType
if errors.As(err, &errRich) {
// It has it, and now errRich is populated.
}
```
**Checking for an exact error value**
Some errors are returned as exact errors such as the [`ErrNotExist`](https://golang.org/pkg/os/#pkg-variables)
error in the `os` package. You can check if this error is present by using
the standard [`errors.Is`](https://golang.org/pkg/errors/#Is) function.
```go
// Assume err is a multierror value
err := somefunc()
if errors.Is(err, os.ErrNotExist) {
// err contains os.ErrNotExist
}
```
**Returning a multierror only if there are errors**
If you build a `multierror.Error`, you can use the `ErrorOrNil` function
to return an `error` implementation only if there are errors to return:
```go
var result *multierror.Error
// ... accumulate errors here
// Return the `error` only if errors were added to the multierror, otherwise
// return nil since there are no errors.
return result.ErrorOrNil()
```