Hacker News Reader

Comment by kazinator | Hacker News Reader

Are you saying that they were easy to implement, and that it was done properly?

Implementing a modern, production-quality compiler is not easy as a baseline, but nothing in the design of OCaml, Haskell, or Rust adds any significant obstacles, relative to Common Lisp, to supporting this feature. Slinging an AST around and dropping it into a quasiquoted template is a well-understood problem. The simplicity of Lisp's syntax is not a prerequisite and hasn't been since the parsing techniques that were developed in the 1970s.

Done properly? I can't speak to camlp4, but at least in the case of Haskell and Rust, certainly. Incidentally I just had my first occasion to write a Rust procedural macro last weekend. I had a significantly complex transformation written and working in half a day, learning curve included, and I found it all pretty frictionless.

The litmus test is being able to remove any phrase structure in the language and replace it by a macro.

Haskell and Rust both pass this. The input doesn't even have to be a production in the source language; you can parse it any way you'd like.

Paul Graham is not a complete idiot, and you are not disproving his statement "hard to implement properly in a language without turning it into a dialect of Lisp".

If you have to parse anything, it's not a Lisp-like macro, which is always an object arising from a production in the language.

Parsing can be hard, so he is right there too. If we take out some subset of Rust that has a significant syntax, and implement it in macros, that sounds like being up to the elbows in parsing.

You're not up to your elbows if somebody else has already done the work for you. If the input to your macro is a valid production in Rust, then all the parsing work that's incumbent on the macro author is to write

    let ast = parse_macro_input!(input as Foo);

Where foo is some type defined by the `syn` crate and there's one for every production in Rust's grammar. But neither are you limited to those. You can also extend or replace that grammar as you see fit, but in that case the added parsing is on you.

Reply on news.ycombinator.com