BitsySwift is a compiler for the Bitsy language implemented in Swift. It is currently the canonical implementation of Bitsy.

Bitsy is a programming language which aims to be the best language to implement when building your first compiler or interpreter. It is a resource for programmers learning about language implementation.

To learn more about Bitsy or to try implementing it yourself, in your favorite language, check out the runnable, test based language specification, BitsySpec.

You can read more about the motivation behind creating Bitsy on the ScopeLift Blog.

I spoke about creating Bitsy and implementing it in Swift at several conferences in 2016. You can watch the talk on YouTube.

This version of BitsySwift has been tested with:

• macOS 10.14 (Mojave) or later
• Xcode 11.3
• Swift 5.0

Linux support is currently limited by Swift Foundation but should come eventually.

To 'install' the compiler, simply clone and build the repository. You must have Xcode and the xcodebuild utility installed.

git clone https://github.com/apbendi/bitsy-swift.git
cd bitsy-swift
./build.sh

Once built, you can use the runbitsy script to conveniently build and immediately run any .bitsy file.

./runbitsy samples/collatz.bitsy # Print the Collatz sequence for 7
22
11
34
17
52
26
13
40
20
10
5
16
8
4
2
1

Note: The runbitsy script currently hangs for Bitsy programs which accept user input, though the binaries themselves run fine. Any bash experts know why?

Alternatively, you may directly use the bitsy-swift command line utility directly for additional options:

bin/bitsy-swift --help
Usage: bin/bitsy-swift [options]
  -v, --version:
      Print the version of bitsy-swift
  -h, --help:
      Display bitsy-swift usage
  -c, --read-cli:
      Read Bitsy code from the command line, terminated by a '.'
  -o, --output:
      Specify a name for the binary output
  -e, --emit-cli:
      Emit intermediate compilation to command line
  -r, --run-delete:
      Immediately run and delete the compiled binary
  -i, --retain-intermediate:
      Retain results of intermediate representation

While Bitsy has been created partially in response to a perceived lack of approachable resources for learning language implementation, there are still some good places to start.

• Let's Build a Compiler; this paper from the late 80's (!) is an excellent introduction to compilation. The biggest downside is the use of Pascal and m68K assembly. While working through this tutorial, I partially translate his code to Swift.
• The Super Tiny Compiler is a great resource by James Kyle- a minimal, extremely well commented compiler written in JavaScript. Be sure to also checkout the associated conference talk
• How to implement a programming language in JavaScript is a slightly more advanced resource which is, for better or worse, also written in JavaScript.
• A Nanopass Framework for Compiler Education (PDF)
• Stanford Compiler Course with Alex Aiken; an advanced resource for learning some theory and going deeper.

Contributions of all types are welcome! Open an issue, create a pull request, or just ask a question. The only requirement is that you be respectful of others.

Please checkout the BitsySpec repo and join the discussion to codify version 1.0 of the Bitsy language specification.