lisp-cpp

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Introduction

This is an under-development implementation of a bytecode compiler in C++ for a subset of Lisp. I'm adapting a similar compiler implemented in Python, described in this blog post by Michael Bernstein. This is an educational project so that I can learn about bytecode compilers/interpreters and develop greater proficiency with C++.

The compiler currently consists of two phases:

1. A bytecode compilation phase that compiles the Lisp AST to a simple Instruction Set Architecture (ISA), consisting of the following instructions:

   • LOAD_CONST: Pushes constants onto the stack.
   • STORE_NAME: Stores values into the environment.
   • LOAD_NAME: Reads values from the environment.
   • CALL_FUNCTION: Calls a function.
   • RELATIVE_JUMP_IF_TRUE: Jumps if the value on top of the stack is true.
   • RELATIVE_JUMP: Jumps.
   • MAKE_FUNCTION: Creates a function object from a code object on the stack and pushes it on the stack.

2. An interpretration phase where the bytecode is evaluated using a stack-based virtual machine.

The compiler currently lacks a frontend, which is hopefully soon to come. Build and run instructions also in the works.

Installation

Dependencies

To build this project you need a C++ compiler (clang or gcc) and the C++ Boost library, which is used for its testing framework and for writing recursive variants. Using the recursive variants only requires the Boost header files, but the testing framework requires dynamic linking to the boost_unit_test_framework.

I installed Boost using homebrew on MacOS. Alternatively, you can follow the installation instructions on getting started with Boost here.

Building the executable

Currently the only way to work with the compiler is by writing test expressions and then compiling/evaluating them, as in test/compiler-test.cpp. To build the test executable, the Makefile needs to be modified to include your compiler path. The argument to the -I flag should also be modified to point to where the include directory of the Boost library lives, and so should the argument to the -L flag to point to directory where the Boost dynamic libraries live.

Once this is done, running make will build the test executable.

If you are using VSCode and clangd (as I have been for this project), then an easy way to configure the project such that clangd can find the Boost library is to create a compile_flags.txt file that specifies the path to the Boost library which needs to be dynamically linked using the -L flag and the Boost header files that need to be included using the -I flag (similar to what is done in the Makefile).

Roadmap

Upcoming features

• Compiling define
• Lexer and parser