roc/examples/hello-world

Hello, World!

To run, cd into this directory and run:

$ cargo run Hello.roc

To run in release mode instead, do:

$ cargo run --release Hello.roc

Design Notes

This demonstrates the basic design of hosts: Roc code gets compiled into a pure function (in this case, a thunk that always returns "Hello, World!") and then the host calls that function. Fundamentally, that's the whole idea! The host might not even have a main - it could be a library, a plugin, anything. Everything else is built on this basic "hosts calling linked pure functions" design.

For example, things get more interesting when the compiled Roc function returns a Task - that is, a tagged union data structure containing function pointers to callback closures. This lets the Roc pure function describe arbitrary chainable effects, which the host can interpret to perform I/O as requested by the Roc program. (The tagged union Task would have a variant for each supported I/O operation.)

In this trivial example, it's very easy to line up the API between the host and the Roc program. In a more involved host, this would be much trickier - especially if the API were changing frequently during development.

The idea there is to have a first-class concept of "glue code" which host authors can write (it would be plain Roc code, but with some extra keywords that aren't available in normal modules - kinda like port module in Elm), and which describe both the Roc-host/C boundary as well as the Roc-host/Roc-app boundary. Roc application authors only care about the Roc-host/Roc-app portion, and the host author only cares about the Roc-host/C boundary when implementing the host.

Using this glue code, the Roc compiler can generate C header files describing the boundary. This not only gets us host compatibility with C compilers, but also Rust FFI for free, because rust-bindgen generates correct Rust FFI bindings from C headers.