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+# go-multierror
+
+[![Build Status](http://img.shields.io/travis/hashicorp/go-multierror.svg?style=flat-square)][travis]
+[![Go Documentation](http://img.shields.io/badge/go-documentation-blue.svg?style=flat-square)][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()
+```