Friday, April 9, 2021




The aim is to prove out the idea of an interpreter where the “front end” (lexical analysis) is in Rust and the “back end” (interpretation) is in C++.

  • We’ll parse and evaluate a simple language of arithmetic expressions;
    • The parser is implemented using nom (a Rust parser combinator library);
  • The “tagless-final” idiom is used to split the front and back ends;
  • The interop between C++ and Rust is expressed using the CXX library.


Additive expression syntax we’ll define by this grammar:

    expr := term ('+' term)*
    term := lit | '-' term | '(' expr ')'
    lit  := digits

The key idea of the program is this: Don’t define an abstract syntax tree type, values of which are produced by parsing. Rather, as parsing unfolds, call functions defined by the the folllowing trait.

pub trait ExprSyn: Clone {
    fn lit(n: i64) -> Self;
    fn neg(t: Self) -> Self;
    fn add(u: Self, v: Self) -> Self;

With that understood, we implement the parser as a Rust library in the following way.

pub mod parse {
    use nom::{
        character::complete::{digit1 as digit, space0 as space},
        combinator::{map, map_res},
        sequence::{delimited, pair, preceded},

    use super::ExprSyn;
    use std::str::FromStr;

    type ParseResult<'a, E> = IResult<&'a str, E>;

    fn lit<E: ExprSyn>(i: &str) -> ParseResult<E> {
        map_res(delimited(space, digit, space), |x| {

    fn neg<E: ExprSyn>(i: &str) -> ParseResult<E> {
        map(delimited(space, preceded(char('-'), term), space), E::neg)(i)

    fn par<E: ExprSyn>(i: &str) -> ParseResult<E> {
        delimited(space, delimited(tag("("), expr, tag(")")), space)(i)

    fn term<E: ExprSyn>(i: &str) -> ParseResult<E> {
        alt((lit, neg, par))(i)

    pub fn expr<E: ExprSyn>(i: &str) -> ParseResult<E> {
        let (i, init) = term(i)?;
        fold_many0(pair(char('+'), term), init, |acc, (_, val): (char, E)| {
            E::add(acc, val)

To wire that up to C++ we need express a CXX “bridge”.

use cxx::SharedPtr;

pub mod ffi {
    unsafe extern "C++" {
        type Cpp_repr;

        fn lit(i: i64) -> SharedPtr<Cpp_repr>;
        fn neg(e: SharedPtr<Cpp_repr>) -> SharedPtr<Cpp_repr>;
        fn add(l: SharedPtr<Cpp_repr>, r: SharedPtr<Cpp_repr>) -> SharedPtr<Cpp_repr>;

    extern "Rust" {
        fn parse(s: String) -> Result<SharedPtr<Cpp_repr>>;

The header file cpp_repr.hpp referenced by the bridge contains these prototypes.

#pragma once

#include <memory>
#include <cstdint>

struct Cpp_repr {
  int64_t expr;

using cpp_repr_t = std::shared_ptr<Cpp_repr>;

cpp_repr_t lit(int64_t i);
cpp_repr_t neg(cpp_repr_t t);
cpp_repr_t add(cpp_repr_t t, cpp_repr_t u);

The existence of that header is enough to finish off the Rust library.

pub type CppRepr_t = SharedPtr<ffi::Cpp_repr>;

impl ExprSyn for CppRepr_t {
    fn lit(i: i64) -> CppRepr_t {
    fn neg(t: CppRepr_t) -> CppRepr_t {
    fn add(t1: CppRepr_t, t2: CppRepr_t) -> CppRepr_t {
        ffi::add(t1, t2)

pub fn parse(s: String) -> Result<CppRepr_t, String> {
    match parse::expr::<CppRepr_t>(s.as_str()) {
        Ok((_s, rep)) => Ok(rep),
        Err(e) => Err(format!("{}", e)),


We put the C++ part of the implementation in cpp_repr.cpp in a separate library.

#include <iostream>

#include "arith_final_tagless/include/cpp_repr.hpp"

cpp_repr_t lit(int64_t i) {
  return std::shared_ptr<Cpp_repr>{new Cpp_repr{i}};

cpp_repr_t neg(cpp_repr_t t) {
  return std::shared_ptr<Cpp_repr>{new Cpp_repr{-t->expr}};

cpp_repr_t add(cpp_repr_t t, cpp_repr_t u) {
  return std::shared_ptr<Cpp_repr>{new Cpp_repr{t->expr + u->expr}};


The REPL in main.cpp brings the Rust and C++ libraries together into an executable.

#include <iostream>

#include "rust/cxx.h" // 'rust::Error'
#include "arith_final_tagless/src/" // 'parse'

int main() {
  char const* prompt = "\n% ";
  std::cout << "Additive expression evalutator (type CTRL+D to exit)" << prompt;

  std::string line;
  while(std::getline(std::cin, line)) {
    try {
      if (auto p = parse(rust::String{line})) {
        std::cout << line << " = " << p->expr << prompt;
    } catch (rust::Error const& e) {
      std::cerr << e.what() << prompt;

  return 0;

That’s it! If this is interesting to you and you’d like to go deeper into the technical details like, how to write the build scripts and so on for a program like this, the source is online here.