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A paradox about typesafety and expressiveness

It’s often argued that dynamic typing is more expressive, and I am, for the most part, willing to go along with that characterization. By nature, a language with dynamic typing places fewer constraints on me, the programmer, and lets me “express myself” more freely. I’m not going to, right now, re-argue the case for static typing, which is anyway quite clear to anyone who has ever written or maintained a decent-sized chunk of code using an IDE. However, I would like to point out a particular sense in which dynamic typing is much less expressive than static typing, and the consequences of that.

(Now, please bear with me for a bit, because I’m going to take a rather scenic route to getting to my real point.)

In a nutshell, dynamic typing is less expressive than static typing in that it doesn’t fully express types. Of course that’s a rather silly tautology. But it’s still worth saying. Why? Because types matter. Even in a dynamically-typed language like Python, Smalltalk, or Ruby, types still matter. To understand and maintain the code, I still need to know the types of things. Indeed, this is true even in weakly-typed JavaScript!

The consequence of this is that dynamically-typed code is far less self-documenting than statically typed code. Quick, what can I pass to the following function? What do I get back from it?

function split(string, separators)

Translated to Ceylon, this function signature is immediately far more understandable:

{String*} split(String string, {Character+} separators)

What this means, of course, is that dynamic typing places a much higher burden on the programmer to conscientiously comment and document things. And to maintain that documentation.

On the other hand, static typing forces me to maintain the correct type annotations on the split() function, even when its implementation changes, even when I’m in a hurry, and my IDE will even help me automatically refactor them. No IDE on Earth offers the same kind of help maintaining comments!

Now consider what else this extra expressiveness buys me. Suppose that Foo and Bar share no interesting common supertype. In any dynamic language on earth, I could write the following code:

class Super {
    function fun() { return Foo(); }
class Sub extends Super {
    function fun() { return Bar(); }

But if I did write such a thing, my team-mates would probably want to throttle me! There’s simply too much potential here for a client calling fun()to only handle the case of Foo, and not realize that sometimes it returns a Bar instead. Generally, most programmers will avoid code like the above, and make sure that fun() always returns types closely related by inheritance.

As a second example, consider a well-known hole in the typesystem of Java: null. In Java, I could write:

class Super {
    public Foo fun() { return Foo(); }
class Sub extends Super {
    public Foo fun() { return null; }

Again, this is something Java developers often avoid, especially for public APIs. Since it’s not part of the signature of fun() that it may return null, and so the caller might just obliviously not handle that case, resulting in an NPE somewhere further down the track, it’s often a good practice to throw an exception instead of returning null from a method belonging to a public API.

Now let’s consider Ceylon.

class Super() {
    shared default Foo? fun() => Foo();
class Sub() extends Super() {
    shared actual Null fun() => null;

Since the fact that fun() can return null is part of its signature, and since any caller is forced by the typesystem to account for the possibility of a null return value, there’s absolutely nothing wrong with this code. So in Ceylon it is often a better practice to define functions or methods to just return null instead of throwing a defensive exception. Thus, we arrive at an apparent paradox:

By being more restrictive in how we handle null, we make null much more useful.

Now, since a “nullable type” in Ceylon is just a special case of a union type, we can generalize this observation to other union types. Consider:

class Super() {
    shared default Foo|Bar fun() => Foo();
class Sub() extends Super() {
    shared actual Bar fun() => Bar();

Again, there’s absolutely nothing wrong with this code. Any client calling Super.fun() is forced to handle both possible concrete return types.

What I’m saying is that we achieved a nett gain in expressiveness by adding static types. Things that would have been dangerously error-prone without static typing have become totally safe and completely self-documenting.

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