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44 lines
2 KiB
Markdown
44 lines
2 KiB
Markdown
# Hello, World!
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To run, `cd` into this directory and run:
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```bash
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$ cargo run Hello.roc
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```
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To run in release mode instead, do:
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```bash
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$ cargo run --release Hello.roc
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```
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## Design Notes
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This demonstrates the basic design of hosts: Roc code gets compiled into a pure
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function (in this case, a thunk that always returns `"Hello, World!"`) and
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then the host calls that function. Fundamentally, that's the whole idea! The host
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might not even have a `main` - it could be a library, a plugin, anything.
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Everything else is built on this basic "hosts calling linked pure functions" design.
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For example, things get more interesting when the compiled Roc function returns
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a `Task` - that is, a tagged union data structure containing function pointers
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to callback closures. This lets the Roc pure function describe arbitrary
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chainable effects, which the host can interpret to perform I/O as requested by
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the Roc program. (The tagged union `Task` would have a variant for each supported
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I/O operation.)
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In this trivial example, it's very easy to line up the API between the host and
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the Roc program. In a more involved host, this would be much trickier - especially
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if the API were changing frequently during development.
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The idea there is to have a first-class concept of "glue code" which host authors
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can write (it would be plain Roc code, but with some extra keywords that aren't
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available in normal modules - kinda like `port module` in Elm), and which
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describe both the Roc-host/C boundary as well as the Roc-host/Roc-app boundary.
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Roc application authors only care about the Roc-host/Roc-app portion, and the
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host author only cares about the Roc-host/C boundary when implementing the host.
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Using this glue code, the Roc compiler can generate C header files describing the
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boundary. This not only gets us host compatibility with C compilers, but also
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Rust FFI for free, because [`rust-bindgen`](https://github.com/rust-lang/rust-bindgen)
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generates correct Rust FFI bindings from C headers.
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