roc/examples/platform-switching/web-assembly-platform

Hello, World!

To run this website, we first compile the app that uses the Wasm platform:

• If you use the nightly roc release:

./roc build --target=wasm32 examples/platform-switching/rocLovesWebAssembly.roc

• If you start from the compiler source code:

# Build roc compiler if you have not done so already
cargo build
target/debug/roc build --target=wasm32 examples/platform-switching/rocLovesWebAssembly.roc

We then move the file:

# Go to the directory where index.html is
cd examples/platform-switching/web-assembly-platform/
# Move the .wasm file so that it's beside index.html
mv ../rocLovesWebAssembly.wasm .

In the directory where index.html is, run any web server on localhost.

For example if you have Python3 on your system, you can use http.server:

python3 -m http.server 8080

Or if you have npm, you can use http-server

npm install -g http-server
http-server

Now open your browser at http://localhost:8080

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!\n") 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.