ErrorValueFAQ

Error Values: Frequently Asked Questions

The Go 2 error values proposal adds functionality to the errors and fmt packages of the standard library for Go 1.13. There is also a compatibility package, golang.org/x/xerrors, for earlier Go versions.

We suggest using the xerrors package for backwards compatibility. When you no longer wish to support Go versions before 1.13, use the corresponding standard library functions.

How should I change my error-handling code to work with the new features?

You need to be prepared that errors you get may be wrapped.

• If you currently compare errors using ==, use xerrors.Is instead. Example:

  if err == io.ErrUnexpectedEOF
  

  becomes

  if xerrors.Is(err, io.ErrUnexpectedEOF)
  

  • Checks of the form if err != nil need not be changed.
  • Comparisons to io.EOF need not be changed, because io.EOF should never be wrapped.

• If you check for an error type using a type assertion or type switch, use xerrors.As instead. Example:

  if e, ok := err.(*os.PathError); ok
  

  becomes

  if e := &os.PathError{}; xerrors.As(err, &e)
  

  • Also use this pattern to check whether an error implements an interface.
  • Rewrite a type switch as a sequence of if-elses.

What formatting verb should I use to display errors?

It depends who you expect to see the error message, and why.

• If you are displaying an error so someone can diagnose a problem in your code, use %+v to display the maximum amount of detail. For example, an error that indicates a bug in your program should be displayed (or at least logged) with %+v.

• If the person who sees the error message is unlikely to know or care about modifying your program, you probably want to use %v (or, equivalently for errors, %s). For example, an error reading a file from a command-line program should probably be displayed with %v.

I am already using fmt.Errorf with %v or %s to provide context for an error. When should I switch to %w?

It's common to see code like

if err := frob(thing); err != nil {
    return fmt.Errorf("while frobbing: %v", err)
}

With the new error features, that code continues to work as before; the only difference is that more information is captured and displayed.

Changing from %v to %w doesn't change how the error is displayed, but it does wrap err, the final argument to the fmt.Errorf call. Now the caller can access err, either by calling Unwrap on the returned error, or by using [x]errors.Is or [x]errors.As (which are merely convenience functions built on top of Unwrap).

So use %w if you want to expose the underlying error to your callers. Keep in mind that doing so may be exposing implementation detail that can constrain the evolution of your code. Callers can depend on the type and value of the error you're wrapping, so changing that error can now break them. For example, if your accessDatabase function uses Go's database/sql package, then it may encounter a sql.ErrTxDone error. If you return that error with fmt.Errorf("accessing DB: %v", err) then callers won't see that sql.ErrTxtDone is part of the error you return. But if you instead return fmt.Errorf("accessing DB: %w", err), then a caller could reasonably write

err := accessDatabase(...)
if xerrors.Is(err, sql.ErrTxDone)

At that point, you must always return sql.ErrTxDone if you don't want to break your clients, even if you switch to a different database package.

How can I add context to a returned error without breaking clients?

Say your code now looks like

return err

and you decide that you want to add more information to err before you return it. If you write

return fmt.Errorf("more info: %v", err)

then you might break your clients, because the identity of err is lost; only its message remains.

You could instead wrap the error by using %w, writing

return fmt.Errorf("more info: %w", err)

This will still break clients who use == or type assertion to test errors. But as we discussed in the first question of this FAQ, consumers of errors should migrate to the Is and As functions. If you can be sure that your clients have done so, then it is not a breaking change to switch from

return err

to

return fmt.Errorf("more info: %w", err)