Namespace: go.std.errors

v1.0

Contents

• Summary
• Index
• Constants
• Variables
• Functions, Macros, and Special Forms
• Types

Summary

Provides a low-level interface to the errors package.

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.

Index

• As
• Is
• New
• Unwrap

Legend

Constant Variable Function Macro Special form Type GoVar Receiver/Method

Constants

Constants are variables with :const true in their metadata. Joker currently does not recognize them as special; as such, it allows redefining them or their values.

(None.)

Variables

(None.)

Functions, Macros, and Special Forms

• As

  Function v1.0

  (As err target)
  

  As finds the first error in err's chain that matches target, and if one is found, 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.
  
  Go input arguments: (err error, target any)
  
  Go returns: bool
  
  Joker input arguments: [^Error err, ^GoObject target]
  
  Joker returns: ^Boolean

• Is

  Function v1.0

  (Is err target)
  

  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. An Is method should only shallowly
  compare err and the target and not call Unwrap on either.
  
  Go input arguments: (err error, target error)
  
  Go returns: bool
  
  Joker input arguments: [^Error err, ^Error target]
  
  Joker returns: ^Boolean

• New

  Function v1.0

  (New text)
  

  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.
  
  Go input arguments: (text string)
  
  Go returns: error
  
  Joker input arguments: [^String text]
  
  Joker returns: ^Error

• Unwrap

  Function v1.0

  (Unwrap err)
  

  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.
  
  Go input arguments: (err error)
  
  Go returns: error
  
  Joker input arguments: [^Error err]
  
  Joker returns: ^Error

Types

(None.)