[Golang (Go) pkg] errors

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errors

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.

Package

Import errors package.

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import "errors"

func New

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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.

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    // error message
	err := errors.New("error msg")
	fmt.Printf("%+v\n", err)    // error msg
	
    // format error message
	err = errors.New(fmt.Sprintf("error %s", "msg"))
	fmt.Printf("%+v\n", err)    // error msg
    
	// format error message
	err = fmt.Errorf("error %s", "msg")
	fmt.Printf("%+v\n", err)    // error msg
}

func Unwrap

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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.

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

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

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err := errors.New("error msg")
fmt.Printf("%+v\n", err)

errWrap := fmt.Errorf("%w", err)	// Wrapping errors with %w
fmt.Printf("%s %t\n", "err == errWrap", (err == errWrap))   // err == errWrap false

errUnwrap := errors.Unwrap(errWrap)
fmt.Printf("%s %t\n", "err == errUnwrap", (err == errUnwrap))   // err == errUnwrap true

returns err.

func Is

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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

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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.

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:

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if errors.Is(err, fs.ErrExist)

is preferable to

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if err == fs.ErrExist

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

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err := errors.New("error msg")
fmt.Printf("%+v\n", err)

errWrap := fmt.Errorf("%w", err)	// Wrapping errors with %w
fmt.Printf("%s %t\n", "err == errWrap", (err == errWrap))   // err == errWrap false

errUnwrap := errors.Unwrap(errWrap)
fmt.Printf("%s %t\n", "err == errUnwrap", (err == errUnwrap))   // err == errUnwrap true
fmt.Printf("%s %t\n", "errUnwrap Is err", errors.Is(errUnwrap, err))    // errUnwrap Is err true

errWrapWrap := fmt.Errorf("%w", errWrap)
fmt.Printf("%s %t\n", "err == errWrapWrap", (err == errWrapWrap))   // err == errWrapWrap false
fmt.Printf("%s %t\n", "errWrapWrap Is err", errors.Is(errWrapWrap, err))    // errWrapWrap Is err true

func As

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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.

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

is preferable to

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

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

References

[1] errors - The Go Programming Language - https://golang.org/pkg/errors/

[2] Working with Errors in Go 1.13 - The Go Blog - https://blog.golang.org/go1.13-errors

[3] Go by Example: Errors - https://gobyexample.com/errors