The run-time distinction

At PyCodeConf I had a very interesting discussion with Nick Coghlan which helped me understand something that had long frustrated me with programming languages. Anyone who’s ever taught a new programmer Java knows this, but perhaps hasn’t understood it for what it is. This thing that I hadn’t been appreciating was the distinction some programming languages make between the language that exists at compile time, and the language that exists at run-time.

Take a look at this piece of Java code:

class MyFirstProgram {
    public static void main(String[] args) {
        System.out.println("Hello World!");

Most people don’t appreciate it, but you’re really writing in two programming languages here, one of these languages has things like class and function declarations, and the other has executable statements (and yes, I realize Java has anonymous classes, they don’t meaningfully provide anything I’m about to discuss).

Compare that with the (approximately) equivalent Python code:

def main():
    print "Hello World"

if __name__ == "__main__":

There’s a very important thing to note here, we have executable statements at the top level, something that’s simply impossible in Java, C, or C++. They make a distinction between the top level and your function’s bodies. It follows from this that the function we’ve defined doesn’t have special status by virtue of being at the top level, we could define a function or write a class in any scope. And this is important, because it gives us the ability to express things like decorators (both class and function).

Another example of this distinction that James Tauber pointed out to me is the import statement. In Python is it a line of executable code which invokes machinery in the VM to find a module and load it into the current namespace. In Java it is an indication to the compiler that a certain symbol is in scope, it’s never executed.

Why would anyone care about this distinction though? It’s clearly possibly to write programs in languages on both ends of the spectrum. It appears to me that the expressiveness of a programming language is really a description of what the distance between the compile time language and the runtime language is. Python stands on one end, with no distinction, whereas C/C++/Java stand on the other, with a grand canyon separating them.

But what about a language in the middle? Much of PyPy’s code is written in a language named RPython. It has a fairly interesting property though, its run-time language is pretty close to Java in semantics, it’s statically typed (though type inferenced), it’s compile time language is Python. In practice this means you get many of the benefits in expressiveness as you do from using Python itself. For example you can write a decorator, or generate a class. A good example of this power is in PyPy’s NumPy implementation. We’re able to create the code for doing all the operations on different dtypes (NumPy’s name for the different datatypes its arrays can represent) dynamically, without needing to resort to code generation or repeating ourselves, we’re able to rely on Python as our compile time (or meta-programming) language. The “in-practice” result of this is that writing RPython feels much more like writing Python than it does like writing Java, even though most of your code is written under the same constraints (albeit without the need to explicitly write types).

The distinction between compile-time and run-time in programming languages results in both more pain for programmers, as well as more arcane structures to explain to new users. I believe new languages going forward should make it a goal to either minimize this difference (as Python does) or outfit languages with more powerful compile-time languages which give them the ability to express meta-programming constructs.