Sunday, November 22, 2009

On Exceptions, Mythological Monsters and Household Appliances




Shit happens
Forrest Gump?

Exceptions are a controversial matter among programmers. We disagree about how to deal with them. We disagree if the should be checked by the compiler. We even disagree if they are useful at all. Sometimes controversy arises from real disagreement: I may argue that test-driven-development is the best way to write software while you might say that formal proofs are the way to go. Both sides can productively debate the issue and present thoughtful arguments either way. I posit that the polemics surrounding exceptions are not of this sort. They are, rather, brought about by the inherent confusion in the mechanism. Exceptions are the Hydra of current programming languages, under a simple banner — to handle exceptional conditions — they mix several mechanisms.

The heads

Let's begin with the humble throw term, which is a sort of procedure exit statement, causing the current routine to throw his hands up in the air, call it quits, and immediately return to the caller. The only other statement with a similar effect is return. Of course when calling return, we must return something! In statically typed languages, this something must have a type: the return type of the function or method. Exceptions muddle the picture, if we think of a throw as a kind of return then what is the type being returned? There can be many answers to this question, but let me put forth a proposal: the "return type" of a "throw" is, for all intents and purposes, a variant type, and that is the second head of the Hydra.

This warrants a small digression. Most functional languages have a way of saying "this type FOO is really either a BAR or a BAZ". So every place in the code that is handed a FOO must check whether it is a BAR or a BAZ and deal appropriately. We call FOO a variant type, and BAR and BAZ are its variants. The syntax for the check is called called a type-case1.  And where have we seen syntax for checking the type of something and then acting on it? Catch clauses!

But of course many languages lack the concept of variant types. I'll argue that in practice, exception handling code in effect greenspuns variant types. First, we can observe that in languages with checked exceptions the declaration of the exceptions a method can throw — the throws clause in Java — is basically a variant type declaration in disguise. Even when working with unchecked exceptions variants lurk behind, encoded through subtyping. Just notice how little polymorphism is exploited in exception class hierarchies. Different exceptions are given different types for the sole reason of being distinguished in catch clauses. Just like variant types.

Ok, time for the next head of the monster. We'll continue to look at the catch clause, but this time we'll look at it as a control structure. As such, it is a strange beast, neither a repetition nor purely a decision structure. Once an exception is thrown, the calling method will stop everything it was doing and jump straight to a catch block. In a way, every exception-throwing method call can be seen as a regular call paired with a conditional jump. Dijkstra may not be rolling in his grave, but he is probably feeling a little tingly.

Since we've talked about control flow, let's direct our attention to the mischievous head of the Hydra: stack unwinding. After a throw happens, if the calling method doesn't care to handle an exception, his caller will have to bare the burden or leave it up to his caller, recursively unwinding the stack until some frame decides it can be bothered to catch the exception and deal with it. This is a pretty powerful feature, sometimes it's even exploited to simulate continuations in languages that lack them.2

One more head left: call stack introspection. Besides unwinding the call stack, exceptions may reify it. It is a limited form of reflection, typically used only to print stack traces to a log someplace where they can be attentively ignored. Common lore says capturing the stack frames is an expensive operation, and so should be executed only on, well, exceptional circumstances.

Exceptional Situations

The confusion bus doesn't stop here, I'm afraid. All the complexity we've examined so far is justified under a deceptively simple banner: to handle exceptional conditions. Hidden in these four words is the vast range of situations that may be interpreted as "exceptional". Like any piece that deals with the subject of exceptions, we can't escape the temptation to suggest a taxonomy. At least this one is short, these are the three main categories of "exceptional situations":

  • Programming error: A traditional example would be code trying to index an array beyond it's bounds. Ideally we would design our abstractions so as to make it impossible to commit such coding errors. From formal grammars to dependent type systems this ideal has been driven much of the research on programming languages over the years. Unfortunately, at least for now, we cannot always evade the need for run-time checks.
  • External failure: Something bad happened to some component not within control of the application. Examples: network cable cut, disk crapped out, database died, sysadmin was drunk and lost a round of UNIX Russian roulette.
  • Invalid user entry: "If an error is possible, someone will make it. The designer must assume that all possible errors will occur and design so as to minimize
    the chance of the error in the first place, or its effects once it gets made. Errors should be easy to detect, they should have minimal consequences, and, if possible, their effects should be reversible." This is from Don Norman's superb The Design of Everyday Things. The book is so good I'm going to slip another quote in here: "The designer shouldn't think of a simple dichotomy between errors and correct behaviour; rather, the entire interaction should be treated as a cooperative endeavor between person and machine, one in which misconceptions can arise on either side"

Let's not forget we are classifying "exceptional situations", not exception types. Here is an example that I hope will make the distinction clear. Take the always popular FileNotFoundException, that is thrown when application code attempts to read a file and the file isn't there: where do we place it on our little taxonomy? Imagine the application is a boring old word processor and the user types in the wrong file name. That obviously is a case of invalid user entry. Now, consider the case of a mail server attempting to open the file where it stores all queued outgoing mail. This file was probably created by the server application itself, maybe during the installation procedure, and its absence would be an indication that something is very borked in the server environment. The same File Not Found event is now clearly an external failure. This matters, because the strategy used for dealing with the three categories is different, as we'll soon see.

The Inevitable Cross Product

Time to look at how the heads of the hydra fare up to our new categories:

Programming errors are, by definition, unexpected. If we aren't expecting them, there is no point in writing code to handle different exception types. What could you do when you know some code tripped a NullPointerException that's different from what you would do if the offence was an IndexOutOfBounds? In other words, the variant types head is useless here. Exceptions triggered by programming errors are usually caught in a generic stack frame, in the bowels of infrastructure code — through the magic of stack unwinding — and dealt with by logging the error type, message, and stack trace. After logging there is little left to do but to roll over and die. In multi-user systems, killing the whole process is not very polite, so we might nuke just the current work item. This is the gist of the fault barrier pattern. Call stack introspection is a very valuable aid to pinpoint the bug. So valuable that I go as far as point its absence as the single most important reason to avoid C for teaching programming to beginners.

Practical strategies for dealing with external failures aren't too different. We tend to exploit stack unwinding to catch the exceptions in a frame deep in infrastructure land and log them in pretty much the same way. But, every so often, we might want to code a different action, such as to retry the operation or to trigger a compensation transaction. In these relatively rare cases it might be useful to deal distinctly with different variants. Another aspect where external failures are distinct from programming errors is that call stack introspection isn't so helpful: if the failure is external, the exact place in the code where it was detected is of little significance. Of course, it is important that the exception carries enough information to enable a system administrator to find the origin of the fault by just looking at the log.

Invalid user input is a different beast altogether. Variants are definitely useful; after all it's important to let the user know if their request was over quota rather than under quota. On run-of-the mill interactive OLTP systems, we handle invalid user entry by providing immediate feedback on what was wrong. But not every case is so simple: in some circumstances it may be important to identify suspicious input and raise security flags; in others, we could offer suggestions for the user to correct the data; in asynchronous interactions, we might want to proceed with the action substituting a default or normalized value for the invalid input; and so on. Altogether, there is a wide range of handling strategies, mostly domain-specific. This is a key observation, as not only handling strategies, but also the detection and the very definition of what values are invalid, are full of domain specific decisions. I would go as far as proposing that the controversial domain exceptions all boil down to cases of invalid user input.

What about the other heads of the monster, are they helpful for invalid user inputs? Call stack introspection is clearly not needed. Moreover, the performance costs of reifying the stack can be detrimental for the not-so-rare instances of invalid input. This isn't just premature optimization, as the conscience of these costs can prevent users from even considering exceptions for the domain logic. What about stack unwinding? I'm afraid I don't really have an answer for this one. On the one hand, domain specific handling logic seems to fit naturally in the calling method. On the other paw, if all we do is inform the user that something happened, a generic code further down the stack is awfully convenient.3 

So what?

We've seen that the feature by the name exceptions is really a hodgepodge of language constructs. Beyond the technical aspects, the very raison d'être for the feature is also muddled. It's as if some crazy nineteenth century inventor decided that since people need to be protected from the heat and, hey, food needs cooling to be preserved as well, why not make buildings like large refrigerators? Just keep the food near the bottom where it's colder, the humans near the top where it's not freezing, and presto! His solution is absurd, but that is because the problem itself is ill-formulated: "to keep stuff cool" is not an useful problem statement.  "To deal with exceptional circumstances" isn't either.

I don't know if the above ramblings are of any practical significance. I certainly don't have any concrete proposal to make things better. My only hope is that language designers will keep in mind context and necessity when devising new constructs.

1 In languages where pattern matching is available, it subsumes type-cases. In fact, one way of thinking about pattern matching is that it is a sophisticated form of type-case.

2 Some languages without full support for exceptions do offer a feature called long jumps, that is very close to what I'm calling stack unwinding.

3 To further complicate the issue, languages offering variant types usually allow different program structuring techniques. For instance, instead of a direct chain of method calls we may thread a monad through a series of functions.

1 comment:

Hugo Corbucci said...

I like Smalltalk squeak's continuation solution for handling exceptions.
It works well for programming errors, doesn't clutter your code and allows to reproduce the exact environment in which the error happened (making it much simpler to understand the problem).
Regarding external failures, it works fairly well too (although I agree something better can surely show up) since you can use the variant type to obtain data about the error and insert it into the normal behavior of your software (like you do on Java or similar others).
Finally, when it comes to user input errors, it is clear to me that it is NOT an exception (more like a rule) and should be handled as part of the normal flow of your software and exceptions been used for such are usually a bad smell!

Anyway, thanks for the post. Good stuff to think about.