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.

Sunday, February 7, 2021

Configuring Cabal Build Flags


Configuring Cabal build flags

It’s always an emergency when I go looking for this information!

Suppose you are building HLint.

mkdir ~/tmp && cd ~/tmp
curl -o hlint-3.2.7.tar.gz
gunzip  hlint-3.2.7.tar.gz && tar xvf  hlint-3.2.7.tar

Left to their own devices, HLint and ghc-lib-parser-ex will default to auto mode meaning, they will decide for themselves if they should depend on ghc-lib-parser or native ghc libraries.

Sometimes it’s desirable to force the situation though and explicitly make them do one or the other. How you do that? The answer is of course package Cabal flags. There are two scenarios: building with stack or building with cabal.

  • stack.yaml

    • Force link with ghc-lib-parser

            ghc-lib: true
            auto: false
            no-ghc-lib: false
    • Force link with native ghc

            ghc-lib: false
            auto: false
            no-ghc-lib: true
  • cabal.project

    • Force link with ghc-lib-parser

        packages: hlint-3.2.7
        package hlint
          flags: +ghc-lib
        package ghc-lib-parser-ex
          flags: -auto -no-ghc-lib
    • Force link with native ghc

        packages: hlint-3.2.7
        package hlint
          flags: -ghc-lib
        package ghc-lib-parser-ex
          flags: -auto +no-ghc-lib

When working with Cabal in the HLint repository one can configure with command line constraints like this:

cabal new-build --constraint "hlint -ghc-lib" --constraint "ghc-lib-parser-ex -auto +no-ghc-lib"

Monday, January 18, 2021

Two things in Rust


Two things in Rust

Two things I needed to learn before Rust made sense to me.

1 Pattern binding modes

I don’t remember reading about this in the book. Default binding modes come into play when non-reference patterns are encountered.


let x = Some(3);
let y: &Option<i32> = &x;
match y {
    Some(a) -> {
    // `y` is deref'd and `a` is bound as `ref a`
    None => {}

The default binding mode starts as move. Each time a reference is matched using a non-reference pattern; it will automatically derefence the vaue and update the default binding mode - If the reference is &val, set the default binding mode to ref - If the reference is &mut val: if the current default binding is ref, it should remain ref. Otherwise, set the current binding mode to ref mut.

Full details are given in the 2005-match-ergonomics rustlang RFC.


  • Example
match (&Some(3)) {
    Some(p) =>
      // This pattern is a "non-reference pattern".
      // Dereference the `&` and shift the default binding
      // mode to `ref`. `p` is read as `ref p` and given type `i32`.
   x => {
     // In this arm, we are still in `move` mode by default, so `x`
     // has type `&Option<32>`
  • Desugared
    match(&Some(3)) {
      &Some(ref p) => {
      x => {

2. Implict Deref Coercisons with Functions and Methods

This is another ergonomics feature that saves on explicit &s and *s when writing function and method calls. When we pass a reference to a function or method call deref implicitly as needed to coerce to the parameter target type.

  • Example:
fn hello(name: &str) {
    println!("Hello, {}", name);

fn main() {
    let m = MyBox::new(String::from("Rust"));
  • Example:
pub struct Point {
    x: Vec<i32>,
    y: Vec<i32>,

impl Point {
    pub fn x(&self) -> &Vec<i32> {
        match self {
            &Point { ref x, .. } => x,

fn use_i32(_: &i32) -> () {}

fn use_vi32(_: &Vec<i32>) -> () {}

fn use_str(_: &str) -> () {}

fn use_strr(_: &&String) -> () {}

fn main() {
    let p: Point = Point {
        x: vec![],
        y: vec![],

    let rp: &Point = &p;
    let rrp: &&Point = &rp;

    println!("p.x = {:?}", rrp.x());

    let _s: &str = "foo";
    let s: String = String::from(_s);
    let bs: Box<String> = Box::new(s.clone());
    let bsr: Box<&String> = Box::new(&s);
    let bi32: Box<Vec<i32>> = Box::new(vec![]);


    let r: &&String = &bsr;
    let r2: &String = r.deref();


    let p = Point {
        x: vec![1, 2, 3],
        y: vec![4, 5, 6],
    match &p {
        Point { x, y } => {
            println!("{:?}, {:?}", x, y);
    println!("{:?}, {:?}", p.x, p.y);