roc/examples/hello-web
2022-02-03 23:55:02 +01:00
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platform remove old Effect module things 2022-02-03 23:55:02 +01:00
.gitignore Updates zig9 to zig 2021-11-09 21:18:18 -06:00
Hello.roc Rename base package to pf everywhere, to match tutorial 2021-12-12 05:57:39 -07:00
index.html emit *.wasm files if the backend is wasm32 2021-09-20 23:07:07 +02:00
README.md README corrections 2021-09-19 11:59:24 +01:00
test-node.js Refactor hello-web example for possible future automated testing 2021-09-19 11:48:31 +01:00

Hello, World!

To run, go to the project home directory and run:

$ cargo run -- build --backend=wasm32 examples/hello-web/Hello.roc

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

cd examples/hello-web
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!") 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.