A new low in programming language design and implementation

The new Wat is the best, most lightweight way to implement a JavaScript-based programming language I have found so far.

Basically, I get away from JS as quickly and painlessly as possible, and start writing the language in itself.

So I define a very small set of primitives on the joint foundation of Kernel-like first-class lexical environments and fexprs and delimited continuations. Fexprs are a great tool for language-oriented programming, and delimited continuations allow me to escape from the browser's (and Node's) async hell and implement any concurrency and effect system I like.

To fexprs I also add macros. When a macro is used as the operator of a form, the form's code gets  changed to the macro's output when the macro is first called, a technique I learned from here. I like macros because they make syntactic abstraction cost-free - with fexprs alone there is always an interpretative overhead. Still, Wat macros, like fexprs, do not work with quoted identifiers, but with first-class values, so many hygiene problems are avoided.

To delimited control I also add classic first-order control (sequential, conditional, loop, throw, catch, finally). This runs on the ordinary JS stack. Only when a continuation is captured does the stack get reified on the heap.

And last but not least, I use a JSON-based syntax for writing the language in itself. At first this was just intended as a quick way to not have to specify a parser for Wat, but I'm starting to like it. It allows Wat to truly be embedded in JavaScript.

Wat does not have a type tagging or object system. It uses the raw JavaScript values.

The whole implementation is roughly 350 lines of JavaScript. After these 350 lines, I can already write Wat in Wat, which is just great.