In the past couple of weeks, I completely rewrote the implementation of Hackett’s type language to improve the integration between the type representation and Racket’s macro system. The new type language effectively implements a way to reuse as much of the Racket macroexpanding infrastructure as possible while expanding a completely custom language, which uses a custom set of core forms. The fundamental technique used to do so is not novel, and it seems to be periodically rediscovered every so often, but it has never been published or documented anywhere, and getting it right involves understanding a great number of subtleties about the Racket macro system. While I cannot entirely eliminate the need to understand those subtleties, in this blog post, I hope to make the secret sauce considerably less secret.
Posts tagged macros
Lisps are not known for infix operators, quite the opposite; infix operators generally involve more syntax and parsing than Lispers are keen to support. However, in Hackett, all functions are curried, and variable-arity functions do not exist. Infix operators are almost necessary for that to be palatable, and though there are other reasons to want them, it may not be obvious how to support them without making the reader considerably more complex.
Fortunately, if we require users to syntactically specify where they wish to use infix expressions, support for infix operators is not only possible, but can support be done without modifying the stock
#lang racket reader. Futhermore, the resulting technique makes it possible for fixity information to be specified locally in a way that cooperates nicely with the Racket macro system, allowing the parsing of infix expressions to be manipulated at compile-time by users’ macros.
Racket ships with
racket/generic, a system for defining generic methods, functions that work differently depending on what sort of value they are supplied. I have made heavy use of this feature in my collections library, and it has worked well for my needs, but that system does have a bit of a limitation: it only supports single dispatch. Method implementations may only be chosen based on a single argument, so multiple dispatch is impossible.
Macros are one of Racket’s flagship features, and its macro system really is state of the art. Of course, it can sometimes be difficult to demonstrate why macros are so highly esteemed, in part because it can be hard to find self-contained examples of using macros in practice. Of course, one thing that macros are perfect for is filling a “hole” in the language by introducing a feature a language lacks, and one of those features in Typed Racket is ADTs.