I’ve had a peculiar kind of awful realization after listening to a C++ talk earlier today. The speaker (Bjarne Stroustrup, actually) went over a few defining components of the language, before he took a much longer stop at templates. In C++14, templates are back in the spotlight because of the new constraints feature, intended to make working with (and especially debugging) templates much more pleasant.
Everyone using C++ templates now is probably well accustomed to the cryptic and excessively long error messages that the compiler spits out whenever you make even the slightest of mistakes. Because of the duck typing semantics of template arguments, those messages are always exposing the internals of a particular template’s implementation. If you, for example, worked with STL implementation from Visual C++, you would recognize the internal
__rb_tree symbol; it appeared very often if you misused the
set containers. Its appearance was at best only remotely helpful at locating the issue – especially if it was inside a multi-line behemoth of an error message.
Before constraints (or “concepts lite”, as they are dubbed) improve the situation, this is arguably the worst part of the C++ language. But alas, C++ is not the only language offering such a poor user experience. As a matter of fact, there is a whole class of languages which are exactly like that – and they won’t change anytime soon.
Yes, I’m talking about the so-called scripting languages in general, and Python in particular. The analogies are striking, too, once you see past the superfluous differences.
Take the before mentioned duck typing as an example. In Python, it is one of the core semantical tenets, a cornerstone of language’s approach to polymorphism. In current C++, this is precisely the cause of page-long, undecipherable compiler errors. You just don’t know whether it’s a duck before you tell it to quack, which usually happens somewhere deep inside the template code.
But wait! Python et al. also have those “compiler errors”. We just call them stacktraces and have interpreters format them in much a nicer, more readable way.
Of course unlike template-related errors in C++, stacktraces tend to be actually helpful. I pose, however, that it’s mostly because we learned to expect them. Studying Python or any other scripting language, we’re inevitably exposed to them at the very early stage, with a steady learning curve that corresponds to the growing complexity of our code.
This is totally different than having the compiler literally throw its innards at you when you try to sort a list of integers.
What I find the most interesting in this whole intellectual exercise is to examine what solutions are offered by both sides of the comparison.
Dynamic languages propose coping mechanisms, at best. You are advised to liberally blanket your code with automated tests so that failing to quack is immediately registered before the duck (er, code) goes live. While some rudimentary static analysis and linting is typically provided, you generally cannot have a reasonable idea whether your code doesn’t fail at the most basic level before you actually run it.
Now, have you ever unit-tested the template specification process that the C++ compiler performs for any of your own templates? Yeah, I thought so. Except for the biggest marvels of template metaprogramming, this may not be something that even crosses your mind. Instead, the established industry practice is simply “don’t do template metaprogramming”.
But obviously, we want to use dynamic languages, and some of us probably want to do template metaprogramming. (Maybe? Just a few? Anyone?…) Since they clearly appear to be similar problems, it’s not very surprising that remedies start to look somewhat alike. C++ is getting concepts in order to impose some rigidity on the currently free-form template arguments. Python is not fully on that path yet but the signs are evident, with the recent adoptions of enums (that I’ve fretted about) as the most prominent example.
If I’m right here, it would be curious to see what lies at the end of this road. In any case, it will probably have been already invented fifty years ago in Lisp.