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74 lines
2.8 KiB
Markdown
74 lines
2.8 KiB
Markdown
# Web REPL
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## Running locally
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### 1. Build the web REPL
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This builds the compiler as a `.wasm` file, and generates JS glue code.
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It will `cargo install` the `wasm-pack` command line tool if you don't already have it.
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```bash
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crates/repl_wasm/build-www.sh
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```
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### 2. Make symlinks to the generated Wasm and JS
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```bash
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mkdir -p www/public/repl
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cd www/public/repl
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ln -s ../../../crates/repl_wasm/build/roc_repl_wasm_bg.wasm
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ln -s ../../../crates/repl_wasm/build/roc_repl_wasm.js
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```
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These symlinks are ignored by Git.
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> This is a bit different from the production build, where we copy all the files to `www/build/`. But for development, it's convenient to have just one copy of files like `www/public/repl/repl.js`. You can make changes, reload your browser to see them, and commit them to Git, without getting mixed up between different copies of the same file.
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### 3. Run a local HTTP server
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Browsers won't load .wasm files over the `file://` protocol, so you need to serve the files in `./www/build/` from a local web server.
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Any server will do, but this example should work on any system that has Python 3 installed:
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```bash
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cd www/public
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python3 -m http.server
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```
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### 4. Open your browser
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You should be able to find the Roc REPL at <http://127.0.0.1:8000/repl> (or whatever port your web server mentioned when it started up.)
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**Warning:** This is work in progress! Not all language features are implemented yet, error messages don't look nice yet, up/down arrows don't work for history, etc.
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## How it works
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- User types text into the HTML `<input />` tag
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- JS detects the `onchange` event and passes the input text to the Roc compiler WebAssembly module
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- Roc compiler WebAssembly module
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- Parses the text (currently just a single line)
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- Type checks
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- Monomorphizes
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- Generates WebAssembly using the development backend (not LLVM)
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- Returns a slice of bytes to JavaScript
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- JavaScript
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- Takes the slice of bytes and creates a `WebAssembly.Instance`
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- Runs the WebAssembly app
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- Gets the memory address of the result and makes a copy of the app's entire memory buffer
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- Passes the result address and the memory buffer to the compiler for analysis
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- Roc compiler WebAssembly module
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- Analyses the bytes of the result, based on the known return type from earlier
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- Traverses the copied memory buffer to find any child values
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- Produces a user-friendly String and passes it to JavaScript
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- JavaScript
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- Displays the input and output text on the web page
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## Related crates
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There are several directories/packages involved here:
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- `www/public/repl/index.html`: The web page with its JavaScript and a build script
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- `crates/repl_wasm`: The Rust crate that becomes the "compiler" WebAssembly module
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- `crates/repl_eval`: REPL logic shared between `crates/repl_cli` and `crates/repl_wasm`
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