Implementing dynamic scope for Fennel and Lua

Summary

I’m continuing my work on fennel-cljlib, my port of clojure.core and some other core libraries, focusing on porting missing functions and features to it. One such feature, which I sometimes miss in Lua and Fennel, is dynamic binding. The Lua VM doesn’t provide dynamic scoping as a language feature, and Fennel itself doesn’t introduce any concepts like Clojure’s Var. However, we can still implement dynamic scoping that works similarly to Clojure and other Lisps using the debug library. Most of the ideas are based on information from the guides at leafo.net. There’s even a “Dynamic scoping in Lua” guide that implements a slightly different version of this feature, requiring variables to be referenced by name via dynamic("var_name") call. While this approach is feasible, I wanted something more in line with how other Lisps work, so let’s explore advancing it further. Luckily for us, Leafo already has all the necessary guides! But first things first. I wanted to delay working on dynamic scoping as much as possible because it is a feature that’s hard to get right. I already have some experience with implementing dynamic scoping for one of my older libraries that implemented a condition system from Common Lisp in Fennel. This library, however, required special syntax to access all of the dynamically bound symbols and thus did not actually require anything fancy for it to work. So what does dynamic binding/scoping mean in a language? If you know about lexical and dynamic scoping and wish to skip this tangent, feel free to do so. In short, lexically scoped variables exist only where their lexical scope allows them to. For example, a variable defined in a block of code will only exist in that block because it is its lexical scope. When working with languages that have higher-order functions, I often find myself in a situation where I want to refactor some code that uses anonymous functions by moving them out and giving them a name. Sometimes it’s possible; sometimes it’s not...