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+# 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` implements the
+[errwrap](https://github.com/hashicorp/errwrap) interface so that it can
+be used with that library, as well.
+
+## 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
+ }
+}
+```
+
+**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()
+```