Where scala left me wanting

n the past few years, I've grown to enjoy more and more programming in a functional style. Even the java code I write nowadays is mostly free of mutable state. But before that, I spent a good chunk of time learning about object orientation and found some great ideas there. Ideas that can carry over to the functional world. One of them is Domain Driven Design, where we strive to have the symbols in the code respresent conncepts in the domain. Of course, I'm oversimplifying DDD, but the ideal of a bijection between the reality our software models and constructs in a programming language is one I believe we should maintain in many, perhaps most, applications.

A natural consequence is that our systems architecture tends to resemble an onion, with the domain model in the center and code to make it interact with the rest of the world surrounding it:

I've labeled the code that interacts with the rest of the world "Adapters", as it should do little more than adapt external representations into concepts the domain model can understand. This architecture is not in any way novel. In fact, it is a version of Alistair Cockburn Hexagonal Architecture. In the common case where the only interactions the domain model has with the rest of the world are through a Database on one end and an User Interface on the other, we've just described the 3-layer architecture that was so popular in the 90s. Some examples of adapters are — to increase our shot at the buzzword bingo — Web MVC Controllers, Repositories or DAOs, message endpoints, GUI listeners, etc., you get the picture.

Since Scala is a truly hybrid language, marrying quite elegantly OO and FP, it would appear to be the perfect vehicle to write this kind of software. I've found this premise to be correct, for the most part. As you might have guessed, there is an exception; (If there weren't, I wouldn't be writing this post, would I?). The problem is in the code for the adapters. As mentioned, they tend to be simple shims, translating some external representation into domain objects. Coding such translations every single time for every adapter instance in every application is very dull. We desperately need our old friend, lady abstraction, to help us out here. In the Java world, she lends us a hand though dynamic reflection, allowing the construction of objects and invocation of methods to be done generically. Unfortunately, Scala has no reflection API, so we have no alternative but to resort to Java reflection, in a sense reverse-engineering the Scala compilation process.

This isn't a major gripe, as Scala constructs tend to map to Java constructs in a straightforward fashion. Also, rumor has it that a Scala reflection API is being developed for 2.8.1. But that's only half the story. Powerful as it is, dynamic reflection alone is not enough to solve the Adapter problem once and for all. We often need to parameterize the translation in some way. For instance, when translating objects to database rows, we must discover the Column names corresponding to object properties. In many cases we can trust convention, for instance we could expect the columns to match exactly the names of the properties. But in other cases there is no option but to somehow configure the translation with an explicit mapping.

 In the Java world this kind of thing used to be done with verbose and annoying XML configuration files. As the language evolved, annotations were introduced, and they are now the main way to configure such translations.

Annotations are a great improvement over external XML files, but they can't be the end of the story. There is the relatively minor issue that annotations pollute the domain model we strive so much to keep clean and organized, specially when the same domain objects will be active in many adapters. Beyond that, there is the larger problem that annotations are just metadata. Sometimes we want to parameterize our adapters in richer ways. Take, for instance, the proposal for a typesafe API for database queries to be added to the new version of the Java Persistence API. It requires a special pre-processor to generate a metamodel that can be used to parameterize a database adapter. 

A better and more generic approach would be to have the language itself provide this metamodel: a kind of static reflection. I would love to see something like this in Scala. Some time ago I toyed with the idea of writing a compiler plugin to provide a static meta-model of Scala classes, but apparently compiler plugins have issues with non-transparent code generation. 

Postscript. As with all matters regarding programming languages on the web, we must tread lightly. I am not ranting against Scala, in fact I rather enjoy it, as can be gleaned from some of the previous posts. I am only relating a very specific domain where I believe the language can be much improved. 

On Exceptions, Mythological Monsters and Household Appliances

Philosophy aims at the logical clarification of thoughts. Philosophy is not a body of doctrine but an activity. A philosophical work consists essentially of elucidations. Philosophy does not result in 'philosophical propositions', but rather in the clarification of propositions. Without philosophy thoughts are, as it were, cloudy and indistinct: its task is to make them clear and to give them sharp boundaries.
Wittgenstein

If language is not correct, then what is said is not what is meant; if what is said is not what is meant, then what must be done remains undone; if this remains undone, morals and art will deteriorate; if justice goes astray, the people will stand about in helpless confusion. Hence there must be no arbitrariness in what is said. This matters above everything.
Confucius

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.