While reviewing some of the comments on my post about parallelism and concurrency, I noticed that the great fallacy about dynamic and static languages continues to hold people in its thrall. So, in the same “everything you know is wrong” spirit, let me try to set this straight: a dynamic language is a straightjacketed static language that affords less rather than more expressiveness. If you’re one of the lucky ones who already understands this, congratulations, you probably went to Carnegie Mellon! For those who don’t, or think that I’m wrong, well let’s have at it. I’m not going to be very technical in this post; the full technical details are available in my book, Practical Foundations for Programming Languages, which is available online.

So-called dynamic languages (“so-called” because I’m going to argue that they don’t exist as a separate class of languages) are perennially popular. From what I can tell, it’s largely a matter of marketing. Hey, we all want to be dynamic, don’t we? A dynamic personality is one that makes things happen, takes risks, has fun, breaks the rules! And who wants to be static? It’s synonymous with boring, after all, and no one wants to be a bore (even if they are, or especially if they are). So, hey, dynamic languages are cool, right? Or, at any rate, cool people like dynamic languages. Static languages are for drips and losers.

That’s one argument, and, to be honest, it’s quite a successful bit of marketing. Like most marketing tactics, it’s designed to confuse, rather than enlighten, and to create an impression, rather than inform.

Another part of the appeal of dynamic languages appears to be that they have acquired an aura of subversion. Dynamic languages fight against the tyranny of static languages, hooray for us! We’re the heroic vanguard defending freedom from the tyranny of typing! We’re real programmers, we don’t need no stinking type system!

Now I’m as susceptible to the seductions of subversion as the next guy (witness this very post), so I can appreciate wanting to strike one for the little guy, overturning the evil establishment. But I’ve got news for you: when it comes to “dynamic typing”, it’s a futile enterprise. Not because the establishment is so powerful. Not because evil always wins. But because the distinction makes no sense. Dynamic typing is but a special case of static typing, one that limits, rather than liberates, one that shuts down opportunities, rather than opening up new vistas. Need I say it? Something can hardly be opposed to that of which it is but a trivial special case. So, give it up, and get with the program! There are ill-defined languages, and there are well-defined languages. Well-defined languages are statically typed, and languages with rich static type systems subsume dynamic languages as a corner case of narrow, but significant, interest.

Wittgenstein said that all philosophical problems are problems of grammar. And so it is here as well. The root of the problem lies in the confusion between a type and a class. We all recognize that it is often very useful to have multiple classes of values of the same type. The prototypical example is provided by the complex numbers. There is one type of complex numbers that represent points in two-dimensional space. In school we encounter two classes of complex numbers, the rectangular and the polar. That is, there are two ways to present a complex number using two different systems of coordinates. They are, of course, interchangeable, because they represent values of the same type. But the form of presentation differs, and some forms are more convenient than others (rectangular for addition, polar for multiplication, for example). We could, of course, choose a “coin of the realm”, and represent all complex numbers in one way, and consider coordinates as just a matter of how a number is given. But it can also be convenient to consider that the type of complex numbers consists of two classes, each of which consists of two real numbers, but interpreted differently according to the coordinate system we are using.

Crucially, the distinction between the two classes of complex number is dynamic in that a given computation may result in a number of either class, according to convenience or convention. A program may test whether a complex number is in polar or rectangular form, and we can form data structures such as sets of complex numbers in which individual elements can be of either form. But this does not conflict with the basic fact that there is but one static type of complex numbers! In type theoretic terms what I am saying is that the type complex is defined to be the sum of two copies of the product of the reals with itself. One copy represents the class of rectangular representations, the other represents the class of polar representations. Being a sum type, we can “dispatch” (that is, case analyze) on the class of the value of the type complex, and decide what to do at run-time. This is no big deal. In fact, it’s quite simple and natural in languages such as ML or Haskell that support sum types. (Languages that don’t are hobbled, and this is part of the confusion.)

What does this have to do with the concept of a dynamic language? The characteristics of a dynamic language are the same, values are classified by into a variety of forms that can be distinguished at run-time. A collection of values can be of a variety of classes, and we can sort out at run-time which is which and how to handle each form of value. Since every value in a dynamic language is classified in this manner, what we are doing is agglomerating all of the values of the language into a single, gigantic (perhaps even extensible) type. To borrow an apt description from Dana Scott, the so-called untyped (that is “dynamically typed”) languages are, in fact, unityped. Rather than have a variety of types from which to choose, there is but one!

And this is precisely what is wrong with dynamically typed languages: rather than affording the freedom to ignore types, they instead impose the bondage of restricting attention to a single type! Every single value has to be a value of that type, you have no choice! Even if in a particular situation we are absolutely certain that a particular value is, say, an integer, we have no choice but to regard it as a value of the “one true type” that is classified, not typed, as an integer. Conceptually, this is just rubbish, but it has serious, tangible penalties. For one, you are depriving yourself of the ability to state and enforce the invariant that the value at a particular program point must be an integer. For another, you are imposing a serious bit of run-time overhead to represent the class itself (a tag of some sort) and to check and remove and apply the class tag on the value each time it is used. (See PFPL for full details of what is involved.)

Now I am fully aware that “the compiler can optimize this away”, at least in some cases, but to achieve this requires one of two things (apart from unreachable levels of ingenuity that can easily, and more profitably, be expressed by the programmer in the first place). Either you give up on modular development, and rely on whole program analysis (including all libraries, shared code, the works), or you introduce a static type system precisely for the purpose of recording inter-modular dependencies. The whole-program approach does not scale, and flies in the face of the very advantage that dynamic languages supposedly have, namely dynamic evolution and development of components. The static type system approach works beautifully, and makes my point nicely.

I am also fully aware that many statically typed languages, particularly the ones widely in commercial use, do not have a sufficiently expressive type system to make it feasible to work dynamically (that is, with multiple classes of values) within a statically typed framework. This is an argument against bad languages, not an argument against type systems. The goal of (static, there is no other kind) type systems is precisely to provide the expressive power to capture all computational phenonema, including dynamic dispatch and dynamically extensible classification, within the rich framework of a static type discipline. More “researchy” languages such as ML or Haskell, have no trouble handling dynamic modes of programming.

Why should you care? Because, I argue, that a fully expressive language is one that supports the delicate interplay between static and dynamic techniques. Languages that force only one perspective on you, namely the dynamic languages, hobble you; languages that admit both modes of reasoning enable you and liberate you from the tyranny of a single type. Let a thousand flowers bloom!

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