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Wacket

Wacket began as a final project for CMSC430: Compilers. It is a collaboration between myself, Stefan Heller, and Peter Geertsema. This project is a compiler for a lisp-inspired language, that targets WebAssembly. The project's code is on GitHub here. It is implemented mainly in Racket, with a runtime system written in Javascript.

The following is a valid program in Wacket, showcasing function definitions and lambda expressions.

#lang wacket
(define (adder n)
  (λ (x)
    (+ x n)))
((adder 5) 10)

A function called adder is defined, with a single argument n. This function returns a lambda expression that itself takes a single argument, x, and adds n to it. In the main body of the program (the final line), the adder function is called with argument 5, and the resulting lambda expression is called with 10. This program would return 15.

Challenges

A somewhat expected, but worse than expected, challenge of this project was building an adequate understanding of WebAssembly, whose docs are thorough and mathematically rigourous... almost to a fault.

The pretty printer, which takes an Abstract Syntax Tree (AST) representation to a readable text format, was relatively straightforward, but tedious. It was probably the most time consuming individual part of the project. This was thoroughly unexpected, and presented a second early-project slowdown.

Methodology

After starting with building our understanding of WebAssembly as described above, we had to build an abstraction (an AST, to be precise) of WebAssembly to build well-formed WebAssembly programs in our compiler. This was followed by the pretty printer, which allowed us to print our well-formed WebAssembly programs to text files.

From there, we started from the most absolute building blocks, and began building up and adding functionality to our language. The first revision of Wacket involved only integers... and nothing else. One at a time, we added very basic things like booleans, characters, logical control flow, an assortment of primitive operators (+, -, etc.), and local variables. Continuing with more complex features, we added strings, I/O, object storage on a heap (cons cells for lists, boxes for pointers), and functions in both regular and lambda form.

Future work

Over summer 2022, we had our first external PR to the project, with someone adding the modulo (%) operator. In the future, I would love to rework the runtime system to use WASI instead of JavaScript, so programs can execute on the command line instead of in the browser. Additionally, I would love to begin work on a standard library to make the language slightly more usable, even if I still don't expect anyone to actually use it.