Damien Neil and Jonathan Amsterdam

17 October 2019

Introduction

Go’s treatment of errors as values has served us well over the last decade. Although the standard library’s support for errors has been minimal—just the errors.New and fmt.Errorf functions, which produce errors that contain only a message—the built-in error interface allows Go programmers to add whatever information they desire. All it requires is a type that implements an Error method:

type QueryError struct { Query string Err error } func (e *QueryError) Error() string { return e.Query + ": " + e.Err.Error() }

Error types like this one are ubiquitous, and the information they store varies widely, from timestamps to filenames to server addresses. Often, that information includes another, lower-level error to provide additional context.

The pattern of one error containing another is so pervasive in Go code that, after extensive discussion, Go 1.13 added explicit support for it. This post describes the additions to the standard library that provide that support: three new functions in the errors package, and a new formatting verb for fmt.Errorf .

Before describing the changes in detail, let's review how errors are examined and constructed in previous versions of the language.

Errors before Go 1.13

Examining errors

Go errors are values. Programs make decisions based on those values in a few ways. The most common is to compare an error to nil to see if an operation failed.

if err != nil { // something went wrong }

Sometimes we compare an error to a known sentinel value, to see if a specific error has occurred.

var ErrNotFound = errors.New("not found") if err == ErrNotFound { // something wasn't found }

An error value may be of any type which satisfies the language-defined error interface. A program can use a type assertion or type switch to view an error value as a more specific type.

type NotFoundError struct { Name string } func (e *NotFoundError) Error() string { return e.Name + ": not found" } if e, ok := err.(*NotFoundError); ok { // e.Name wasn't found }

Adding information

Frequently a function passes an error up the call stack while adding information to it, like a brief description of what was happening when the error occurred. A simple way to do this is to construct a new error that includes the text of the previous one:

if err != nil { return fmt.Errorf("decompress %v: %v", name, err) }

Creating a new error with fmt.Errorf discards everything from the original error except the text. As we saw above with QueryError , we may sometimes want to define a new error type that contains the underlying error, preserving it for inspection by code. Here is QueryError again:

type QueryError struct { Query string Err error }

Programs can look inside a *QueryError value to make decisions based on the underlying error. You'll sometimes see this referred to as "unwrapping" the error.

if e, ok := err.(*QueryError); ok && e.Err == ErrPermission { // query failed because of a permission problem }

The os.PathError type in the standard library is another example of one error which contains another.

Errors in Go 1.13

The Unwrap method

Go 1.13 introduces new features to the errors and fmt standard library packages to simplify working with errors that contain other errors. The most significant of these is a convention rather than a change: an error which contains another may implement an Unwrap method returning the underlying error. If e1.Unwrap() returns e2 , then we say that e1 wraps e2 , and that you can unwrap e1 to get e2 .

Following this convention, we can give the QueryError type above an Unwrap method that returns its contained error:

func (e *QueryError) Unwrap() error { return e.Err }

The result of unwrapping an error may itself have an Unwrap method; we call the sequence of errors produced by repeated unwrapping the error chain.

Examining errors with Is and As

The Go 1.13 errors package includes two new functions for examining errors: Is and As .

The errors.Is function compares an error to a value.

// Similar to: // if err == ErrNotFound { … } if errors.Is(err, ErrNotFound) { // something wasn't found }

The As function tests whether an error is a specific type.

// Similar to: // if e, ok := err.(*QueryError); ok { … } var e *QueryError if errors.As(err, &e) { // err is a *QueryError, and e is set to the error's value }

In the simplest case, the errors.Is function behaves like a comparison to a sentinel error, and the errors.As function behaves like a type assertion. When operating on wrapped errors, however, these functions consider all the errors in a chain. Let's look again at the example from above of unwrapping a QueryError to examine the underlying error:

if e, ok := err.(*QueryError); ok && e.Err == ErrPermission { // query failed because of a permission problem }

Using the errors.Is function, we can write this as:

if errors.Is(err, ErrPermission) { // err, or some error that it wraps, is a permission problem }

The errors package also includes a new Unwrap function which returns the result of calling an error's Unwrap method, or nil when the error has no Unwrap method. It is usually better to use errors.Is or errors.As , however, since these functions will examine the entire chain in a single call.

Wrapping errors with %w

As mentioned earlier, it is common to use the fmt.Errorf function to add additional information to an error.

if err != nil { return fmt.Errorf("decompress %v: %v", name, err) }

In Go 1.13, the fmt.Errorf function supports a new %w verb. When this verb is present, the error returned by fmt.Errorf will have an Unwrap method returning the argument of %w , which must be an error. In all other ways, %w is identical to %v .

if err != nil { // Return an error which unwraps to err. return fmt.Errorf("decompress %v: %w", name, err) }

Wrapping an error with %w makes it available to errors.Is and errors.As :

err := fmt.Errorf("access denied: %w", ErrPermission) ... if errors.Is(err, ErrPermission) ...

Whether to Wrap

When adding additional context to an error, either with fmt.Errorf or by implementing a custom type, you need to decide whether the new error should wrap the original. There is no single answer to this question; it depends on the context in which the new error is created. Wrap an error to expose it to callers. Do not wrap an error when doing so would expose implementation details.

As one example, imagine a Parse function which reads a complex data structure from an io.Reader . If an error occurs, we wish to report the line and column number at which it occurred. If the error occurs while reading from the io.Reader , we will want to wrap that error to allow inspection of the underlying problem. Since the caller provided the io.Reader to the function, it makes sense to expose the error produced by it.

In contrast, a function which makes several calls to a database probably should not return an error which unwraps to the result of one of those calls. If the database used by the function is an implementation detail, then exposing these errors is a violation of abstraction. For example, if the LookupUser function of your package pkg uses Go's database/sql package, then it may encounter a sql.ErrNoRows error. If you return that error with fmt.Errorf("accessing DB: %v", err) then a caller cannot look inside to find the sql.ErrNoRows . But if the function instead returns fmt.Errorf("accessing DB: %w", err) , then a caller could reasonably write

err := pkg.LookupUser(...) if errors.Is(err, sql.ErrNoRows) …

At that point, the function must always return sql.ErrNoRows if you don't want to break your clients, even if you switch to a different database package. In other words, wrapping an error makes that error part of your API. If you don't want to commit to supporting that error as part of your API in the future, you shouldn't wrap the error.

It’s important to remember that whether you wrap or not, the error text will be the same. A person trying to understand the error will have the same information either way; the choice to wrap is about whether to give programs additional information so they can make more informed decisions, or to withhold that information to preserve an abstraction layer.

Customizing error tests with Is and As methods

The errors.Is function examines each error in a chain for a match with a target value. By default, an error matches the target if the two are equal. In addition, an error in the chain may declare that it matches a target by implementing an Is method.

As an example, consider this error inspired by the Upspin error package which compares an error against a template, considering only fields which are non-zero in the template:

type Error struct { Path string User string } func (e *Error) Is(target error) bool { t, ok := target.(*Error) if !ok { return false } return (e.Path == t.Path || t.Path == "") && (e.User == t.User || t.User == "") } if errors.Is(err, &Error{User: "someuser"}) { // err's User field is "someuser". }

The errors.As function similarly consults an As method when present.

Errors and package APIs

A package which returns errors (and most do) should describe what properties of those errors programmers may rely on. A well-designed package will also avoid returning errors with properties that should not be relied upon.

The simplest specification is to say that operations either succeed or fail, returning a nil or non-nil error value respectively. In many cases, no further information is needed.

If we wish a function to return an identifiable error condition, such as "item not found," we might return an error wrapping a sentinel.

var ErrNotFound = errors.New("not found") // FetchItem returns the named item. // // If no item with the name exists, FetchItem returns an error // wrapping ErrNotFound. func FetchItem(name string) (*Item, error) { if itemNotFound(name) { return nil, fmt.Errorf("%q: %w", name, ErrNotFound) } // ... }

There are other existing patterns for providing errors which can be semantically examined by the caller, such as directly returning a sentinel value, a specific type, or a value which can be examined with a predicate function.

In all cases, care should be taken not to expose internal details to the user. As we touched on in "Whether to Wrap" above, when you return an error from another package you should convert the error to a form that does not expose the underlying error, unless you are willing to commit to returning that specific error in the future.

f, err := os.Open(filename) if err != nil { // The *os.PathError returned by os.Open is an internal detail. // To avoid exposing it to the caller, repackage it as a new // error with the same text. We use the %v formatting verb, since // %w would permit the caller to unwrap the original *os.PathError. return fmt.Errorf("%v", err) }

If a function is defined as returning an error wrapping some sentinel or type, do not return the underlying error directly.

var ErrPermission = errors.New("permission denied") // DoSomething returns an error wrapping ErrPermission if the user // does not have permission to do something. func DoSomething() error { if !userHasPermission() { // If we return ErrPermission directly, callers might come // to depend on the exact error value, writing code like this: // // if err := pkg.DoSomething(); err == pkg.ErrPermission { … } // // This will cause problems if we want to add additional // context to the error in the future. To avoid this, we // return an error wrapping the sentinel so that users must // always unwrap it: // // if err := pkg.DoSomething(); errors.Is(err, pkg.ErrPermission) { ... } return fmt.Errorf("%w", ErrPermission) } // ... }

Conclusion

Although the changes we’ve discussed amount to just three functions and a formatting verb, we hope they will go a long way toward improving how errors are handled in Go programs. We expect that wrapping to provide additional context will become commonplace, helping programs to make better decisions and helping programmers to find bugs more quickly.

As Russ Cox said in his GopherCon 2019 keynote, on the path to Go 2 we experiment, simplify and ship. Now that we’ve shipped these changes, we look forward to the experiments that will follow.