# Rust Call Lean

A tiny example of calling Lean's FFI from Rust with Nix.

This is based on:
* [Lean4 Reverse FFI](https://github.com/leanprover/lean4/tree/master/src/lake/examples/reverse-ffi)
* [Lean4 FFI](https://lean-lang.org/lean4/doc/dev/ffi.html#initialization)
* [Rustonomicon FFI](https://doc.rust-lang.org/nomicon/ffi.html)
* [Rust Bindgen](https://rust-lang.github.io/rust-bindgen/introduction.html)
* [Rust Bindgen Static Functions](https://github.com/rust-lang/rust-bindgen/discussions/2405)

## Usage

Since the entire environment was built on Nix flakes, the easiest way to run the executable is via Nix:
```
$ nix run .#caller
```
For interactive development, one can also drop into a develop shell:
```
$ nix develop
```
and then execute
```
$ cargo run
```
Execution without Nix is possible.

This project has been tested on Linux and Mac OS.

## Structure

The project has 2 parts:
1. `Callee`, a Lean library
2. `Caller`, a Rust executable

`Caller` is dependent on `Callee` and requires it along with Lean to be present
in the environment for linking. Specifically the following environment variables
must be set:
* `CALLEE_PATH`: Path to a directory containing `libCallee.so`
* `LEAN_ROOT`: Path to a directory such that `$LEAN_ROOT/include` has lean
  headers and `$LEAN_ROOT/lib` has the `leanshared` library
* Clang path must be set so `clang-sys` can find it
* Clang executable must be present in `PATH`