README

multierr GoDoc Build Status Coverage Status

multierr allows combining one or more Go errors together.

Installation

go get -u go.uber.org/multierr

Status

Stable: No breaking changes will be made before 2.0.


Released under the MIT License.

Expand ▾ Collapse ▴

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())

This makes it possible to record resource cleanup failures from deferred blocks with the help of named return values.

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

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

Examples

Constants

This section is empty.

Variables

This section is empty.

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())
    	}()
    
    Example
    Output:
    
    call 1 failed; call 2 failed
    

    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 verison 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)
      }
      
      Example
      Output:
      
      call 1 failed
      call 3 failed
      foo; baz
      

      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))
        
        Example
        Output:
        
        the following errors occurred:
         -  call 1 failed
         -  call 3 failed
         -  call 5 failed
        

        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.

          Example
          Output:
          
          call 2 failed
          call 3 failed
          call 5 failed
          

          Types

          This section is empty.

          Source Files