bf2kernel

What's Kernel?

Dart is a multiplatform language that can be interpreted or be compiled to JS (with two different compilers!) and native code. With that many backends, you don't want to write an end-to-end compiler for each of them. Some work, like parsing, type checking and resolving methods is completely independent of the backend, so why do that multiple times? In Dart, a package called front_end is responsible to compile Dart into an intermediate format called Kernel. All compilers then take a Kernel component to do their work. For instance, when you run dart some/script.dart, what's actually happening is that

• the front_end package will read script.dart and its depencies and compile them to Kernel
• the vm package (not published to pub) will apply some VM-specific optimizations and create bytecode only understood by the Dart VM
• the VM then runs that bytecode, JIT-compiling parts to native code on the way

Since all compilers work with Kernel components, we can make use of all Dart backends if we write a front_end equivalent for Brainfuck.

Note: You might have heard about the analyzer package, which is the base for static analysis and Dart IDEs. It also shares some parts with the frontend (such as the parser), but is entirely independent otherwise.

What's Brainfuck?

Brainfuck is a very simple language, which makes it suitable to experiment with compilers. At runtime, a brainfuck program state consists of two variables:

• an array of integers to store data. It's up to us to define how large this should be, we'll use an Int16List from dart:typed_data with a lenght of ten thousand. We'll call this array data from now on.
• a single pointer which refers to an index into the array.

With that, a Brainfuck program is just a sequence of these characters:

• > increments the pointer by one
• < decrements the pointer by one
• + increments data[pointer] by one
• - decrements data[pointer] by one
• . outputs data[pointer] to stdout
• , reads a byte from stdin and stores it at data[pointer]

Here's a simple BF program, can you guess what it's doing?

,[.,]

The first , will read a char from stdin and write it to data[0]. As long as data[0] is not zero, we'll output that char and read another one. So, this program will simply output it's input and stop when it's fed a \0 char.

This compiler

The compiler mainly consists of three files:

• lib/src/parser.dart: Parse a Brainfuck
• lib/src/instructions.dart: Classes to represent a Brainfuck program
• lib/src/codegen.dart: Compiles a Brainfuck program into an existing Kernel file from the Dart SDK

Another file, bin/main.dart loads the platform Kernel from the Dart SDK and glues the steps together.

Usage

After cloning this repo, you can run

$ dart bin/main.dart path/to/file.bf

It will write a out.dill into the current directory. That file can be run with dart out.dill.