multierr

Documentation

Overview

Package multierr allows combining one or more errors together.

Overview

Errors can be combined with the use of the Combine function.

multierr.Combine(
	reader.Close(),
	writer.Close(),
	conn.Close(),
)

If only two errors are being combined, the Append function may be used instead.

err = multierr.Append(reader.Close(), writer.Close())

The underlying list of errors for a returned error object may be retrieved with the Errors function.

errors := multierr.Errors(err)
if len(errors) > 0 {
	fmt.Println("The following errors occurred:", errors)
}

Appending from a loop

You sometimes need to append into an error from a loop.

var err error
for _, item := range items {
	err = multierr.Append(err, process(item))
}

Cases like this may require knowledge of whether an individual instance failed. This usually requires introduction of a new variable.

var err error
for _, item := range items {
	if perr := process(item); perr != nil {
		log.Warn("skipping item", item)
		err = multierr.Append(err, perr)
	}
}

multierr includes AppendInto to simplify cases like this.

var err error
for _, item := range items {
	if multierr.AppendInto(&err, process(item)) {
		log.Warn("skipping item", item)
	}
}

This will append the error into the err variable, and return true if that individual error was non-nil.

See AppendInto for more information.

Deferred Functions

Go makes it possible to modify the return value of a function in a defer block if the function was using named returns. This makes it possible to record resource cleanup failures from deferred blocks.

func sendRequest(req Request) (err error) {
	conn, err := openConnection()
	if err != nil {
		return err
	}
	defer func() {
		err = multierr.Append(err, conn.Close())
	}()
	// ...
}

multierr provides the Invoker type and AppendInvoke function to make cases like the above simpler and obviate the need for a closure. The following is roughly equivalent to the example above.

func sendRequest(req Request) (err error) {
	conn, err := openConnection()
	if err != nil {
		return err
	}
	defer multierr.AppendInvoke(err, multierr.Close(conn))
	// ...
}

See AppendInvoke and Invoker for more information.

Advanced Usage

Errors returned by Combine and Append MAY implement the following interface.

type errorGroup interface {
	// Returns a slice containing the underlying list of errors.
	//
	// This slice MUST NOT be modified by the caller.
	Errors() []error
}

Note that if you need access to list of errors behind a multierr error, you should prefer using the Errors function. That said, if you need cheap read-only access to the underlying errors slice, you can attempt to cast the error to this interface. You MUST handle the failure case gracefully because errors returned by Combine and Append are not guaranteed to implement this interface.

var errors []error
group, ok := err.(errorGroup)
if ok {
	errors = group.Errors()
} else {
	errors = []error{err}
}

Index

• func Append(left error, right error) error
• func AppendInto(into *error, err error) (errored bool)
• func AppendInvoke(into *error, invoker Invoker)
• func Combine(errors ...error) error
• func Errors(err error) []error
• type Invoke
  • func (i Invoke) Invoke() error
• type Invoker
  • func Close(closer io.Closer) Invoker
• type MultiError
  • func (merr *MultiError) Append(err error)
  • func (merr *MultiError) As(target interface{}) bool
  • func (merr *MultiError) Error() string
  • func (merr *MultiError) Errors() []error
  • func (merr *MultiError) Format(f fmt.State, c rune)
  • func (merr *MultiError) HasErrors() bool
  • func (merr *MultiError) Is(target error) bool

Functions

func Append

func Append(left error, right error) error

Append appends the given errors together. Either value may be nil.

This function is a specialization of Combine for the common case where there are only two errors.

err = multierr.Append(reader.Close(), writer.Close())

The following pattern may also be used to record failure of deferred operations without losing information about the original error.

func doSomething(..) (err error) {
	f := acquireResource()
	defer func() {
		err = multierr.Append(err, f.Close())
	}()

func AppendInto

func AppendInto(into *error, err error) (errored bool)

AppendInto appends an error into the destination of an error pointer and returns whether the error being appended was non-nil.

var err error
multierr.AppendInto(&err, r.Close())
multierr.AppendInto(&err, w.Close())

The above is equivalent to,

err := multierr.Append(r.Close(), w.Close())

As AppendInto reports whether the provided error was non-nil, it may be used to build a multierr error in a loop more ergonomically. For example:

var err error
for line := range lines {
	var item Item
	if multierr.AppendInto(&err, parse(line, &item)) {
		continue
	}
	items = append(items, item)
}

Compare this with a version that relies solely on Append:

var err error
for line := range lines {
	var item Item
	if parseErr := parse(line, &item); parseErr != nil {
		err = multierr.Append(err, parseErr)
		continue
	}
	items = append(items, item)
}

func AppendInvoke

func AppendInvoke(into *error, invoker Invoker)

AppendInvoke appends the result of calling the given Invoker into the provided error pointer. Use it with named returns to safely defer invocation of fallible operations until a function returns, and capture the resulting errors.

func doSomething(...) (err error) {
	// ...
	f, err := openFile(..)
	if err != nil {
		return err
	}

	// multierr will call f.Close() when this function returns and
	// if the operation fails, its append its error into the
	// returned error.
	defer multierr.AppendInvoke(&err, multierr.Close(f))

	scanner := bufio.NewScanner(f)
	// Similarly, this scheduled scanner.Err to be called and
	// inspected when the function returns and append its error
	// into the returned error.
	defer multierr.AppendInvoke(&err, multierr.Invoke(scanner.Err))

	// ...
}

Without defer, AppendInvoke behaves exactly like AppendInto.

err := // ...
multierr.AppendInvoke(&err, mutltierr.Invoke(foo))

// ...is roughly equivalent to...

err := // ...
multierr.AppendInto(&err, foo())

The advantage of the indirection introduced by Invoker is to make it easy to defer the invocation of a function. Without this indirection, the invoked function will be evaluated at the time of the defer block rather than when the function returns.

// BAD: This is likely not what the caller intended. This will evaluate
// foo() right away and append its result into the error when the
// function returns.
defer multierr.AppendInto(&err, foo())

// GOOD: This will defer invocation of foo unutil the function returns.
defer multierr.AppendInvoke(&err, multierr.Invoke(foo))

multierr provides a few Invoker implementations out of the box for convenience. See Invoker for more information.

func Combine

func Combine(errors ...error) error

Combine combines the passed errors into a single error.

If zero arguments were passed or if all items are nil, a nil error is returned.

Combine(nil, nil)  // == nil

If only a single error was passed, it is returned as-is.

Combine(err)  // == err

Combine skips over nil arguments so this function may be used to combine together errors from operations that fail independently of each other.

multierr.Combine(
	reader.Close(),
	writer.Close(),
	pipe.Close(),
)

If any of the passed errors is a multierr error, it will be flattened along with the other errors.

multierr.Combine(multierr.Combine(err1, err2), err3)
// is the same as
multierr.Combine(err1, err2, err3)

The returned error formats into a readable multi-line error message if formatted with %+v.

fmt.Sprintf("%+v", multierr.Combine(err1, err2))

func Errors

func Errors(err error) []error

Errors returns a slice containing zero or more errors that the supplied error is composed of. If the error is nil, a nil slice is returned.

err := multierr.Append(r.Close(), w.Close())
errors := multierr.Errors(err)

If the error is not composed of other errors, the returned slice contains just the error that was passed in.

Callers of this function are free to modify the returned slice.

Types

type Invoke

type Invoke func() error

Invoke wraps a function which may fail with an error to match the Invoker interface. Use it to supply functions matching this signature to AppendInvoke.

For example,

func processReader(r io.Reader) (err error) {
	scanner := bufio.NewScanner(r)
	defer multierr.AppendInvoke(&err, multierr.Invoke(scanner.Err))
	for scanner.Scan() {
		// ...
	}
	// ...
}

In this example, the following line will construct the Invoker right away, but defer the invocation of scanner.Err() until the function returns.

defer multierr.AppendInvoke(&err, multierr.Invoke(scanner.Err))

func (Invoke) Invoke

func (i Invoke) Invoke() error

Invoke calls the supplied function and returns its result.

type Invoker

type Invoker interface {
	Invoke() error
}

Invoker is an operation that may fail with an error. Use it with AppendInvoke to append the result of calling the function into an error. This allows you to conveniently defer capture of failing operations.

See also, Close and Invoke.

func Close

func Close(closer io.Closer) Invoker

Close builds an Invoker that closes the provided io.Closer. Use it with AppendInvoke to close io.Closers and append their results into an error.

For example,

func processFile(path string) (err error) {
	f, err := os.Open(path)
	if err != nil {
		return err
	}
	defer multierr.AppendInvoke(&err, multierr.Close(f))
	return processReader(f)
}

In this example, multierr.Close will construct the Invoker right away, but defer the invocation of f.Close until the function returns.

defer multierr.AppendInvoke(&err, multierr.Close(f))

type MultiError

type MultiError struct {
	// contains filtered or unexported fields
}

MultiError is an error that holds one or more errors.

An instance of this is guaranteed to be non-empty and flattened. That is, none of the errors inside MultiError are other multiErrors.

MultiError formats to a semi-colon delimited list of error messages with %v and with a more readable multi-line format with %+v.

func (*MultiError) Append

func (merr *MultiError) Append(err error)

func (*MultiError) As

func (merr *MultiError) As(target interface{}) bool

As attempts to find the first error in the error list that matches the type of the value that target points to.

This function allows errors.As to traverse the values stored on the multierr error.

func (*MultiError) Error

func (merr *MultiError) Error() string

func (*MultiError) Errors

func (merr *MultiError) Errors() []error

Errors returns the list of underlying errors.

This slice MUST NOT be modified.

func (*MultiError) Format

func (merr *MultiError) Format(f fmt.State, c rune)

func (*MultiError) HasErrors

func (merr *MultiError) HasErrors() bool

func (*MultiError) Is

func (merr *MultiError) Is(target error) bool

Is attempts to match the provided error against errors in the error list.

This function allows errors.Is to traverse the values stored on the multierr error.