Posts tagged racket

Macroexpand anywhere with local-apply-transformer!

⦿ racket, macros

Racket programmers are accustomed to the language’s incredible capacity for extension and customization. Writing useful macros that do complicated things is easy, and it’s simple to add new syntactic forms to meet domain-specific needs. However, it doesn’t take long before many budding macrologists bump into the realization that only certain positions in Racket code are subject to macroexpansion.

To illustrate, consider a macro that provides a Clojure-style let form:

(require syntax/parse/define)

(define-simple-macro (clj-let [{~seq x:id e:expr} ...] body:expr ...+)
  (let ([x e] ...) body ...))

This can be used anywhere an expression is expected, and it does as one would expect:

> (clj-let [x 1
            y 2]
    (+ x y))
3

However, a novice macro programmer might realize that clj-let really only modifies the syntax of binding pairs for a let form. Therefore, could one define a macro that only adjusts the binding pairs of some existing let form instead of expanding to an entire let? That is, could one write the above example like this:

(define-simple-macro (clj-binding-pairs [{~seq x:id e:expr} ...])
  ([x e] ...))

> (let (clj-binding-pairs
        [x 1
         y 2])
    (+ x y))
3

The answer is no: the binding pairs of a let form are not subject to macroexpansion, so the above attempt fails with a syntax error. In this blog post, we will examine the reasons behind this limitation, then explain how to overcome it using a solution that allows macroexpansion anywhere in a Racket program.

Custom core forms in Racket, part II: generalizing to arbitrary expressions and internal definitions

⦿ racket, macros

In my previous blog post, I covered the process involved in creating a small language with a custom set of core forms. Specifically, it discussed what was necessary to create Hackett’s type language, which involved expanding to custom expressions. While somewhat involved, Hackett’s type language was actually a relatively simple example to use, since it only made use of a subset of the linguistic features Racket supports. In this blog post, I’ll demonstrate how that same technique can be generalized to support runtime bindings and internal definitions, two key concepts useful if intending to develop a more featureful language than Hackett’s intentionally-restrictive type system.

Reimplementing Hackett’s type language: expanding to custom core forms in Racket

⦿ racket, hackett, macros

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.

A space of their own: adding a type namespace to Hackett

As previously discussed on this blog, my programming language, Hackett, is a fusion of two languages, Haskell and Racket. What happens when two distinctly different programming languages collide? Hackett recently faced that very problem when it came to the question of namespacing: Haskell has two namespaces, one for values and another for types, but Racket is a staunch Lisp–1 with a single namespace for all bindings. Which convention should Hackett adopt?

For now, at least, the answer is that Hackett will emulate Haskell: Hackett now has two namespaces. Of course, Hackett is embedded in Racket, so what did it take to add an entirely new namespace to a language that possesses only one? The answer was a little more than I had hoped, but it was still remarkably simple given the problem: after two weeks of hacking, I’ve managed to get something working.

Hackett progress report: documentation, quality of life, and snake

⦿ hackett, racket, haskell

Three months ago, I wrote a blog post describing my new, prototype implementation of my programming language, Hackett. At the time, some things looked promising—the language already included algebraic datatypes, typeclasses, laziness, and even a mini, proof of concept web server. It was, however, clearly still rather rough around the edges—error messages were poor, features were sometimes brittle, the REPL experience was less than ideal, and there was no documentation to speak of. In the time since, while the language is still experimental, I have tackled a handful of those issues, and I am excited to announce the first (albeit quite incomplete) approach to Hackett’s documentation.

I’d recommend clicking that link above and at least skimming around before reading the rest of this blog post, as its remainder will describe some of the pieces that didn’t end up in the documentation: the development process, the project’s status, a small demo, and some other details from behind the scenes.

User-programmable infix operators in Racket

⦿ racket, hackett, 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.

Realizing Hackett, a metaprogrammable Haskell

Almost five months ago, I wrote a blog post about my new programming language, Hackett, a fanciful sketch of a programming language from a far-off land with Haskell’s type system and Racket’s macros. At that point in time, I had a little prototype that barely worked, that I barely understood, and was a little bit of a technical dead-end. People saw the post, they got excited, but development sort of stopped.

Then, almost two months ago, I took a second stab at the problem in earnest. I read a lot, I asked a lot of people for help, and eventually I got something sort of working. Suddenly, Hackett is not only real, it’s working, and you can try it out yourself!

Rascal is now Hackett, plus some answers to questions

Since I published my blog post introducing Rascal, I’ve gotten some amazing feedback, more than I had ever anticipated! One of the things that was pointed out, though, is that Rascal is a language that already exists. Given that the name “Rascal” came from a mixture of “Racket” and “Haskell”, I always had an alternative named planned, and that’s “Hackett”. So, to avoid confusion as much as possible, Rascal is now known as Hackett.

With that out of the way, I also want to answer some of the other questions I received, both to hopefully clear up some confusion and to have something I can point to if I get the same questions in the future.

Climbing the infinite ladder of abstraction

I started programming in elementary school.

When I was young, I was fascinated by the idea of automation. I loathed doing the same repetitive task over and over again, and I always yearned for a way to solve the general problem. When I learned about programming, I was immediately hooked: it was so easy to turn repetitive tasks into automated pipelines that would free me from ever having to do the same dull, frustrating exercise ever again.

Of course, one of the first things I found out once I’d started was that nothing is ever quite so simple. Before long, my solutions to eliminate repetition grew repetitive, and it became clear I spent a lot of time typing out the same things, over and over again, creating the very problem I had initially set out to destroy. It was through this that I grew interested in functions, classes, and other repetition-reducing aids, and soon enough, I discovered the wonderful world of abstraction.