Calling hell from heaven and heaven from hell is a classic paper from the previous century, introducing the Haskell foreign function interface (FFI). It describes the facilities that Haskell offers for calling functions written in C (and vice versa). In this blog post, we will consider how to call functions written in Rust instead: not quite hell, but not quite heaven either.

We will make use of two libraries that we wrote to streamline this process: a Haskell-side library called foreign-rust, and a Rust-side library called haskell-ffi. We developed these libraries as part of the development of Be, a (smart) contract platform; we are thankful to them for making these libraries open source. That said, this blog post should also be useful for people who do not want to use these libraries, and indeed, we will also show examples of interop that do not rely on them. The source code for the examples discussed in this blog post can be found at https://github.com/well-typed/blogpost-purgatory.

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