Think C++ template metaprogramming but much, much easier and thus, seemingly more powerful. It's not that you couldn't do the same in C++, but D's metaprogramming is so much more accessible that it makes you want to use it.
You'd probably do large portions of it in "constexpr" types and functions rather than relying on recursive metaprogramming techniques.
They are also planning on adding overloading based on whether a function is constexpr, which is important if you want the same library to support both compile-time and run-time matchers.
So it's all theoretically possible already. But D has the advantage here in that it's not just possible but usable.
Well, imagine that all of the things you can do in Python by messing with dunder methods, function decorators, or metaclasses could be precomputed by a compiler and would not even execute at all during runtime. It makes everything really fast at runtime and easy to precompute at compile time.
Is there anything in D similar to decorators? How does the registration pattern work in D, for instance?
https://github.com/rejectedsoftware/vibe.d/blob/master/examp...
Decorators (in D, 'user defined attributes', or UDA) work differently. It's not a function-composition feature, but more of a tagging feature. You write a class, function, etc., tag it with custom attributes; and then have a separate compile-time function walk over your code, find the decorators, and augment the code based the meaning of the tags. (I used to wish that D had adopted Python-style decorators, since they are easy to reason about and implement, but I can see the logic of the more general UDA system that they adopted.)
In practice, though, you would often use templates to achieve the same effect. Given a memoize template (really, just a memoize function), and an expensive-computation function,
auto fastComputer = memoize!expensiveComputer;
@memoize
def expensiveComputer(): ...