[Cython] [ENH] Compile-time code execution (macros / templates)

[Prakhar Goel]

> Indeed, internally it rewrites the AST, of course. I think that's the
> simplest way to implement it. But it doesn't mean the user that write
> the compile-time code would have to be aware of the AST at all.
> I mean, if you have a good use-case for it, then I'm ok with allowing
> the compile-time code manipulating it. But I'd really prefer this not be
> the main use of the compile-time execution feature.

Ah. I see where you are coming from. Wouldn't this be a ridiculous
amount of work? At least if we insisted on something with a clean
mental model that wouldn't just shred the user with a thousand corner
cases? It would need a whole new sub-syntax in Cython with detailed
semantics for all the weird things code lets people do (e.g. what if
the input function f gets used like a first-class object and placed in
a Python list that then gets passed to another function... at compile
time...). You're basically asking for scheme's syntax-case but harder
because Python/Cython are nowhere near as regular! And scheme's
syntax-case is a beast (read up on phase separation if you don't
believe me: https://docs.racket-lang.org/guide/phases.html).

AST manipulation isn't _that_ hard (esp. with good helper libraries)
and on the plus side, the basic idea is dead-simple to implement: just
call some random Python function that the user points you towards.
There's going to be some scaffolding but of an entirely manageable
amount.

I mean... if you think "there's really no major obstacle" then more
power to you! I'd love to see what you come up with. But I think
you're underestimating the complexity of the task.
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Warm Regards
Prakhar Goel