Ruby has that... But you're then just implementing your own transpiler or interpreter in full, instead of "just" redefining parts of the grammar. I'm not sold in this approach, but it's an interesting read at least.

I know of multiple ways to do it in perl - 'use Foo;' can introduce a source filter that slurps the entire remaining file text, thus hiding it from the perl compiler.

__DATA__ sooort of counts and is probably the closest to pure STOPPARSE as GP is thinking of.

I also, once, wrote https://p3rl.org/Devel::Declare which basically grabbed the current read position and Did Things, including calling back into the existing perl parser to not have to re-implement everything - that was how perlers first got access to a mostly* working 'method' keyword.

(now there's a proper way of doing that in perl core, and Devel::Declare is (very happily to me) obsolete, but it was essential to prove the concept and the user interest to justify adding the actual feature to the core interpreter)

[*] the caveats are many and believe me I'm well aware the entire thing was a giant hack from the moment I first started writing it, but rather than trying to write them out I'll just say "however hacky you imagine this was, what I actually did is probably worse" and leave it there.

Yes, although then at that point you'd be writing your own parser and interpreter basically, which Cognition automates to some extent

Not really, not as most people (I think) would understand the term.

That just gives you arbitrary input. You can write your own interpreter for a language and do the input in that language, but that’s not “arbitrary metaprogramming” in the host language, at least as most people understand it. Or you can leverage whatever metaprogramming facilities are offered by the host via an external DSL you implement, which may be a more ergonomic way of metaprogramming the host language, but still is limited to the metaprogramming facilities exposed by the host language, so it’s only “arbitrary metaprogramming” to the extent that the host language already exposes arbitrary metaprogramming.