Arroyo.cc / tech / Don't be afraid to panic

In Go, functions that can fail return an error value, that will be non-nil if something went wrong. It is up to the caller of a function to handle any errors that may occur. In the wild, such Go code looks like this:

x, err := strconv.Atoi(input)
if err != nil {
	// handle or return the error
}

While I have never found this too troublesome, if your most common way to handle an error is by returning it, you will type more in Go than you do in a language with exceptions, such as python. There are a few techinques described in the blog post "Errors are values" that can help reduce some of the repetition.

Go has a seldom-used exception system in the form of three functions: defer, panic, and recover. These three functions work together to provide something like exceptions for Go:

It is not as flexible as the exception systems in other languages, and is intended for truly exceptional errors, or programmer mistakes, where the best response is to crash the program. However, sometimes the behavior of defer, panic, and recover can be useful for handling more routine errors. When developing the xsd package, I defined a walk function to walk a tree of xml elements:

func (el *xmltree.Element) Walk(func (*xmltree.Element) error) error

I quickly found that every time I encountered an error, I would immediately stop parsing and "bubble up" the error, adding annotations along the way:

var result []Element
err := root.Walk(func(el *xmltree.Element) {
	...
	var v Element
	max := el.Attr("maxOccurs")
	if max == "unbounded" {
		v.Plural = true
	} else if max != "" {
		i, err := strconv.Atoi(max)
		if err != nil {
			return fmt.Errorf("Invalid maxOccurs %q: %v",
				max, err)
		}
		v.Plural = (i > 1)
	}
	result = append(result, v)
	return nil
})
return result, err

I soon found the repetitive error checking overwhelming. I quickly rewrote the code to use panic.

func stop(msg string) {
	panic(parseError{message: msg})
}

func walk(root *xmltree.Element, fn func(*xmltree.Element)) {
	defer func() {
		if r := recover(); r != nil {
			if err, ok := r.(parseError); ok {
				err.path = append(err.path, root)
				panic(err)
			} else {
				panic(r)
			}
		}
	}()
	for i := 0; i < len(root.Children); i++ {
		fn(&root.Children[i])
	}
}

// defer catchParseError(&err)
func catchParseError(err *error) {
	if r := recover(); r != nil {
		*err = r.(parseError)
	}
}

I then wrote several wrappers around library functions that would panic instead of returning an error:

func parseInt(s string) int {
	switch s {
	case "":
		return 0
	case "unbounded":
		return -1
	}
	n, err := strconv.Atoi(s)
	if err != nil {
		stop(err.Error())
	}
	return n
}

The result was less cluttered parsing code:

var result []Element
walk(root, func(el *xmltree.Element) {
	var v Element
	if max := parseInt(el.Attr("maxOccurs")); max < 0 || max > 1 {
		v.Plural = true
	}
	result = append(result, v)
})
return result

This works because in this use case, all errors are handled in the same way, and because recursion is heavily used while parsing due to the deep nesting present in XML schema documents. Because all of the parsing functions went from returning a result and an error, to returning a single result, the functions became much more composable, almost like commands in a unix pipeline. Compare:

// With explicit error returns
elem, err := parseElement(root)
if err != nil {
	return fmt.Errorf("Error parsing element %s: %v", root.Name.Local, err)
}
t.Elements = append(t.Elements, elem)

// With "exceptions"
t.Elements = append(t.Elements, parseElement(root))

While the conventional advice around defer, panic and recover is not to use them, recognizing cases where they are appropriate can lead to clearer and more concise code.