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...and some examples of the rule in action.
This post will describe a simple rule for writing error messages that I've been using for some time and have found to be worthwhile. Using this rule I can be sure that my errors are propagated upwards with everything needed to debug problems, while not containing tons of extraneous or duplicate information.
This rule is not specific to any particular language, pattern of error propagation (e.g. exceptions, signals, simple strings), or method of embedding information in errors (e.g. key/value pairs, formatted strings).
I do not claim to have invented this system, I'm just describing it.
Without more ado, here's the rule:
A function sending back an error should not include information the caller
could already know.
Pretty simple, really, but the best rules are. Keeping to this rule will result in error messages which, once propagated up to their final destination (usually some kind of logger), will contain only the information relevant to the error itself, with minimal duplication.
The reason this rule works in tandem with good encapsulation of function behavior. The caller of a function knows only the inputs to the function and, in general terms, what the function is going to do with those inputs. If the returned error only includes information outside of those two things then the caller knows everything it needs to know about the error, and can continue on to propagate that error up the stack (with more information tacked on if necessary) or handle it in some other way.
(For examples I'll use Go, but as previously mentioned this rule will be useful in any other language as well.)
Let's go through a few examples, to show the various ways that this rule can manifest in actual code.
In this example we have a function which merely wraps a call to `io.Copy` for two files:
func copyFile(dst, src *os.File) error {
_, err := io.Copy(dst, src)
return err
}
In this example there's no need to modify the error from `io.Copy` before returning it to the caller. What would we even add? The caller already knows which files were involved in the error, and that the error was encountered during some kind of copy operation (since that's what the function says it does), so there's nothing more to say about it.
In this example we will open a file, read its contents, and return them as a string:
func readFile(path string) (string, error) {
f, err := os.Open(path)
if err != nil {
return "", fmt.Errorf("opening file: %w", err)
}
defer f.Close()
contents, err := io.ReadAll(f)
if err != nil {
return "", fmt.Errorf("reading contents: %w", err)
}
return string(contents), nil
}
In this example there are two different steps which could result in an error: opening the file and reading its contents. If an error is returned then our imaginary caller doesn't know which step the error occurred at. Using our rule we can infer that it would be good to annotate at *which* step the error is from, so the caller is able to have a fuller picture of what went wrong.
Note that each annotation does *not* include the file path which was passed into the function. The caller already knows this path, so an error being returned back which reiterates the path is unnecessary.
In this example we will read two files using our function from example 2, and return the concatenation of their contents as a string.
func concatFiles(pathA, pathB string) (string, error) {
contentsA, err := readFile(pathA)
if err != nil {
return "", fmt.Errorf("reading contents of %q: %w", pathA, err)
}
contentsB, err := readFile(pathB)
if err != nil {
return "", fmt.Errorf("reading contents of %q: %w", pathB, err)
}
return contentsA + contentsB, nil
}
Like in example 2 we annotate each error, but instead of annotating the action we annotate which file path was involved in each error. This is because if we simply annotated with the string `reading contents` like before it wouldn't be clear to the caller *which* file's contents couldn't be read. Therefore we include which path the error is relevant to.
In this example we will show how using this rule habitually results in easy to read errors which contain all relevant information surrounding the error. Our example reads one file, the "full" file, using our `readFile` function from example 2. It then reads the concatenation of two files, the "split" files, using our `concatFiles` function from example 3. It finally determines if the two strings are equal:
func verifySplits(fullFilePath, splitFilePathA, splitFilePathB string) error {
fullContents, err := readFile(fullFilePath)
if err != nil {
return fmt.Errorf("reading contents of full file: %w", err)
}
splitContents, err := concatFiles(splitFilePathA, splitFilePathB)
if err != nil {
return fmt.Errorf("reading concatenation of split files: %w", err)
}
if fullContents != splitContents {
return errors.New("full file's contents do not match the split files' contents")
}
return nil
}
As previously, we don't annotate the file paths for the different possible errors, but instead say *which* files were involved. The caller already knows the paths, there's no need to reiterate them if there's another way of referring to them.
Let's see what our errors actually look like! We run our new function using the following:
err := verifySplits("full.txt", "splitA.txt", "splitB.txt")
fmt.Println(err)
Let's say `full.txt` doesn't exist, we'll get the following error:
reading contents of full file: opening file: open full.txt: no such file or directory
The error is simple, and gives you everything you need to understand what went wrong: while attempting to read the full file, during the opening of that file, our code found that there was no such file. In fact, the error returned by `os.Open` contains the name of the file, which goes against our rule, but it's the standard library so what can ya do?
Now, let's say that `splitA.txt` doesn't exist, then we'll get this error:
reading concatenation of split files: reading contents of "splitA.txt": opening file: open splitA.txt: no such file or directory
Now we did include the file path here, and so the standard library's failure to follow our rule is causing us some repitition. But overall, within the parts of the error we have control over, the error is concise and gives you everything you need to know what happened.
As with all rules, there are certainly exceptions. The primary one I've found is that certain helper functions can benefit from bending this rule a bit. For example, if there is a helper function which is called to verify some kind of user input in many places, it can be helpful to include that input value within the error returned from the helper function:
func verifyInput(str string) error {
if err := check(str); err != nil {
return fmt.Errorf("input %q was bad: %w", str, err)
}
return nil
}
`str` is known to the caller so, according to our rule, we don't need to include it in the error. But if you're going to end up wrapping the error returned from `verifyInput` with `str` at every call site anyway it can be convenient to save some energy and break the rule. It's a trade-off, convenience in exchange for consistency.
Another exception might be made with regards to stack traces.
In the set of examples given above I tended to annotate each error being returned with a description of where in the function the error was being returned from. If your language automatically includes some kind of stack trace with every error, and if you find that you are generally able to reconcile that stack trace with actual code, then it may be that annotating each error site is unnecessary, except when annotating actual runtime values (e.g. an input string).
As in all things with programming, there are no hard rules; everything is up to interpretation and the specific use-case being worked on. That said, I hope what I've laid out here will prove generally useful to you, in whatever way you might try to use it.
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Published 2021-03-20