Documentation

Overview

    Package errors implements functions to manipulate errors.

    The New function creates errors whose only content is a text message.

    The Unwrap, Is and As functions work on errors that may wrap other errors. An error wraps another error if its type has the method

    Unwrap() error
    

    If e.Unwrap() returns a non-nil error w, then we say that e wraps w.

    Unwrap unpacks wrapped errors. If its argument's type has an Unwrap method, it calls the method once. Otherwise, it returns nil.

    A simple way to create wrapped errors is to call fmt.Errorf and apply the %w verb to the error argument:

    errors.Unwrap(fmt.Errorf("... %w ...", ..., err, ...))
    

    returns err.

    Is unwraps its first argument sequentially looking for an error that matches the second. It reports whether it finds a match. It should be used in preference to simple equality checks:

    if errors.Is(err, fs.ErrExist)
    

    is preferable to

    if err == fs.ErrExist
    

    because the former will succeed if err wraps fs.ErrExist.

    As unwraps its first argument sequentially looking for an error that can be assigned to its second argument, which must be a pointer. If it succeeds, it performs the assignment and returns true. Otherwise, it returns false. The form

    var perr *fs.PathError
    if errors.As(err, &perr) {
    	fmt.Println(perr.Path)
    }
    

    is preferable to

    if perr, ok := err.(*fs.PathError); ok {
    	fmt.Println(perr.Path)
    }
    

    because the former will succeed if err wraps an *fs.PathError.

    Example
    Output:
    
    1989-03-15 22:30:00 +0000 UTC: the file system has gone away
    

    Index

    Examples

    Constants

    This section is empty.

    Variables

    This section is empty.

    Functions

    func As added in go1.13

    func As(err error, target interface{}) bool

      As finds the first error in err's chain that matches target, and if so, sets target to that error value and returns true. Otherwise, it returns false.

      The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.

      An error matches target if the error's concrete value is assignable to the value pointed to by target, or if the error has a method As(interface{}) bool such that As(target) returns true. In the latter case, the As method is responsible for setting target.

      An error type might provide an As method so it can be treated as if it were a different error type.

      As panics if target is not a non-nil pointer to either a type that implements error, or to any interface type.

      Example
      Output:
      
      Failed at path: non-existing
      

      func Is added in go1.13

      func Is(err, target error) bool

        Is reports whether any error in err's chain matches target.

        The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.

        An error is considered to match a target if it is equal to that target or if it implements a method Is(error) bool such that Is(target) returns true.

        An error type might provide an Is method so it can be treated as equivalent to an existing error. For example, if MyError defines

        func (m MyError) Is(target error) bool { return target == fs.ErrExist }
        

        then Is(MyError{}, fs.ErrExist) returns true. See syscall.Errno.Is for an example in the standard library.

        Example
        Output:
        
        file does not exist
        

        func New

        func New(text string) error

          New returns an error that formats as the given text. Each call to New returns a distinct error value even if the text is identical.

          Example
          Output:
          
          emit macho dwarf: elf header corrupted
          
          Example (Errorf)

            The fmt package's Errorf function lets us use the package's formatting features to create descriptive error messages.

            Output:
            
            user "bimmler" (id 17) not found
            

            func Unwrap added in go1.13

            func Unwrap(err error) error

              Unwrap returns the result of calling the Unwrap method on err, if err's type contains an Unwrap method returning error. Otherwise, Unwrap returns nil.

              Types

              This section is empty.

              Source Files