This is so damn cool. And most people won’t even see why.

They won’t realize just how amazingly cool it is that you can grab any block of code in a block closure (the [ ] syntax*), and extend that object with more behavior. And how if you have that object you can I introspect the craziest things out of it, including of course, the source it was derived from, and hence, an AST. There’s no need to make a new compiler pass, or a special tool mod. The “reserved” keyword of any doesn’t have to be bolted into the whole language, it’s just a variable that can be treated specially in this context, and only this context, so you don’t have to have special meta variables leak into the whole system.

* disclaimer: I participated in the inclusion of John and Dons excellent work into the VisualWorks IDE back in the day. I also saw “once” introduced into the Smalltalk system, a lazy runtime way of memoizing block values in place. Oh the things you could do with BlockClosure extension methods.

* aside, my favorite closure syntax of all time, way better than elixirs shorthand & stuff, or fn -> things, or trailing closures