Combining Rust and Haskell

Rust is a pretty interesting language, in the area of C++ but more modern / better. The stated goal of rust are: "a systems programming language focused on three goals: safety, speed, and concurrency". Combining Rust with Haskell could create some interesting use cases, and could replace use of C in some projects while providing a more high level and safer approach where Haskell cannot be used.

One of my reason for doing this, is that writing code targetting low-level features is simpler in Rust than Haskell. For example, writing inline assembly or some lowlevel OS routines. Also the performance of Rust is quite close to C++, and I could see this being useful in certain case where Haskell is not as optimised.

In this short tutorial, let's call Rust functions from Haskell.

The Rust library§

First we start with an hypothetical rust library that takes a value, print to console and return a value.

Our entry point in Rust is a simple rust_hello, in a src/lib.rs file:

#[no_mangle]
pub extern fn rust_hello(v: i32) -> i32 {
    println!("Hello Rust World: {}", v);
    v+1
} 

One of the key thing here is the presence of the no_mangle pragma, that allow exporting the name of the function as-is in the library we're going to generate.

Rust uses Cargo to package library and executable, akin to Cabal for haskell. We can create the Cargo.toml for our test library:

[package]
name = "hello"
version = "0.0.1"
authors = ["Vincent Hanquez <vincent@snarc.org>"]

[lib]
name = "hello"
crate-type = ["staticlib"]

The only special trick is that we ask Cargo to build a static library in the crate-type section, instead of the default rust lib (.rlib).

Haskell doesn't know other calling / linking convention like C++ (yet) or Rust, which is why we need to go through those hoops.

We can now build with our Rust library with:

cargo build

If everything goes according to plan, you should end up with a target directory where you can find the libhello.a library.

The haskell part§

Now the haskell part is really easy, as this point there's no much difference than linking with some static C library; first we create a src/Main.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
module Main where

import Foreign.C.Types

foreign import ccall unsafe "rust_hello" rust_hello :: CInt -> IO CInt

main = do
    v <- rust_hello 1234
    putStrLn ("Rust returned: " ++ show v)

Nothing special if you've done some C bindings yourself, otherwise I suggest having a look at the FFI article.

we can try directly linking with ghc:

ghc -o hello-rust --make src/Main.hs -lhello -Ltarget/debug

and running:

$ ./hello-rust
Hello Rust World: 1234
Rust returned: 1235

That achieve the goal above. From there we can polish things and add this to a cabal file:

name:                hello-rust
version:             0.1.0.0
license:             PublicDomain
license-file:        LICENSE
author:              Vincent Hanquez
maintainer:          vincent@snarc.org
category:            System
build-type:          Simple
cabal-version:       >=1.10
extra-source-files:  src/lib.rs

executable hello-rust
  main-is:             Main.hs
  other-extensions:    ForeignFunctionInterface
  build-depends:       base >=4.8
  hs-source-dirs:      src
  default-language:    Haskell2010
  extra-lib-dirs:      target/release, target/debug
  extra-libraries:     hello

Note: The target/release path is to support building with the -release flag of cargo build. by listing the target/release and then target/debug, it should allow you to pickup the release library in preference to the debug library. It can also create some confusion, and print a warning on my system when one of the directory is missing.

The missing step is either adding some pre-build rules to cabal Setup.hs to run cargo build, or some more elaborate build system, both which are left as exercice to the interested reader.

Where this could go§

Going forward, this could lead to having another language from Haskell to target that is not as lowlevel as C, but offer stellar performance and more high level constructs (than C) without imposing any other runtime system. This is interesting where you need to complete some tasks that Haskell is not quite ready to handle (yet).

For example, a non exhaustive list:

• Writing cryptographic bindings in rust&asm instead of C&asm
• Heavily Vector-Optimised routines
• Operating system routines (e.g. page table handling) for a hybrid and safer operating system kernel.
• Servo embedding

Let me know in the comments anything else that might be of interests !