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roc/crates/cli_testing_examples/platform-switching/web-assembly-platform/README.md
Jan Van Bruggen 527f39b8f2
Move Roc CLI testing examples to crates/
2022-09-11 22:32:15 -06:00

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Hello, World!

To run this website, first compile either of these identical apps:

# Option A: Compile crates/cli_testing_examples/platform-switching/rocLovesWebAssembly.roc
cargo run -- build --target=wasm32 crates/cli_testing_examples/platform-switching/rocLovesWebAssembly.roc

# Option B: Compile crates/cli_testing_examples/platform-switching/main.roc with `pf: "web-assembly-platform/main.roc"` and move the result
cargo run -- build --target=wasm32 crates/cli_testing_examples/platform-switching/main.roc
(cd crates/cli_testing_examples/platform-switching && mv rocLovesPlatforms.wasm web-assembly-platform/rocLovesWebAssembly.wasm)

Then cd into the website directory and run any web server that can handle WebAssembly. For example, with http-server:

cd crates/cli_testing_examples/platform-switching/web-assembly-platform
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.