"What I find interesting about this is the way he did codegen by passing in mock objects that just emit llvm it in the simplest possible way and that it actually works while being incredibly succinct."
It's interesting, but it doesn't work in general, which is why you don't see the technique in common use. The problem is that it only catches methods called on objects, but it can't catch anything that isn't a method called on an object. Unfortunately, that includes for loops, if statements, some aspects of exceptions, and just generally "all flow control".
So rather than being a powerful and general technique, it's a powerful, but very very specific technique that only works in certain toy problems. Unfortunately, once you leave those toy problems behind, it doesn't even help you solve the problem because the paradigm this forces you to operate under makes easy things easy, but medium things hard and hard things pull-your-hair-out impossible. You're better off writing a real compiler... or doing this in a language that does expose enough of what's going on to make this technique work, although you tend to end up in either a Lisp or Haskell, which come at this from very different angles but both have the capacity to make this work. (Lisp via syntax tree manipulation, Haskell via Free Monads and other similar techniques.)
It's interesting, but it doesn't work in general, which is why you don't see the technique in common use. The problem is that it only catches methods called on objects, but it can't catch anything that isn't a method called on an object. Unfortunately, that includes for loops, if statements, some aspects of exceptions, and just generally "all flow control".
So rather than being a powerful and general technique, it's a powerful, but very very specific technique that only works in certain toy problems. Unfortunately, once you leave those toy problems behind, it doesn't even help you solve the problem because the paradigm this forces you to operate under makes easy things easy, but medium things hard and hard things pull-your-hair-out impossible. You're better off writing a real compiler... or doing this in a language that does expose enough of what's going on to make this technique work, although you tend to end up in either a Lisp or Haskell, which come at this from very different angles but both have the capacity to make this work. (Lisp via syntax tree manipulation, Haskell via Free Monads and other similar techniques.)