new post: A playground for Hare
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_posts/2024-06-04-playground-for-hare.md
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permalink: "/{{ year }}/{{ month }}/{{ day }}/playground-for-hare"
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title: "A playground for Hare"
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published_date: "2024-06-04 22:59:00 +0200"
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layout: post.liquid
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data:
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route: blog
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excerpt: |
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I built an online for the programming language Hare in Hare.
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Why? Because I wanted to do something fun and learn Hare.
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How? By using some existing libraries and writing the rest myself.
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---
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I built a thing:
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<center>
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### [Hare Playground][playground]
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</center>
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And this is what it looks like:[^1]
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It's an online playground for the [Hare programming language][harelang].
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Code is sent to the server for compilation and execution.
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It responds with the standard and error output serialized into JSON, which gets rendered in the output box in the frontend.
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This works just the same as [other](https://go.dev/play/) [playgrounds](https://play.rust-lang.org/).
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It's a bit slow and might crash on you, but it works (if it doesn't: reload).
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## Why?
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Because I wanted to do something fun and learn Hare.
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Hare is a rather new systems programming language (started in 2020) that's inspired by C,
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but promises to do few things better.
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The original blog post [Hare's advances compared to C][advances-on-c] has a good overview on that.
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This project gave me the opportunity to explore a couple of areas in Hare,
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read through the documentation and the Hare codebase and even fix a few bugs in existing code.
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I might write another more detailed blog post on my impressions.
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For now I can say that it was a few fun nights learning and exploring a new language.
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## How?
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The whole codebase is [in the hare-playground repository][hare-playground].
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In about 500 lines of my own code plus 3 external libraries I implemented the webserver part,
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including routing and static file serving.
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I bet it has memory leaks.
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```c
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fn route_request(buf: *io::stream, serv_req: *http::server_request) (void | http_err) = {
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let request = &serv_req.request;
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switch (request.method) {
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case "GET" =>
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switch (request.target.path) {
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case "/" => handle_index(buf, serv_req);
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case =>
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handle_file(buf, serv_req)?;
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return;
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};
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case "POST" =>
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switch (request.target.path) {
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case "/v1/exec" => handle_exec(buf, serv_req)?;
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case => return notfound;
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};
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case "OPTIONS" =>
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handle_cors(serv_req);
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return;
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case => return notfound;
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};
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};
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```
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> The "router" part of the webserver[^2]
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The external libraries I pulled in are:
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1. [hare-logfmt] for formatted logging messages (though I only log the minimum).
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1. [hare-json] for parsing from and serializing to JSON (unmodified from upstream).
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1. [hare-http] for the webserver parts: listening on a socket, parsing HTTP, creating the request and response objects.
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I added a couple of commits to fix some issues (available in [my fork](https://git.fnordig.de/jer/hare-http/commits/branch/host-in-uri), I'll upstream them).
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I also wrote a [small wrapper around pthread](https://git.fnordig.de/jer/hare-playground/src/branch/main/vendor/hare-thread/thread/thread.ha),
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just so my server can handle more than one request at a time.
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To run the user-submitted Hare code the server launches `hare` in a [bubblewrap][] sandbox.
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This should make it somewhat safe to deal with arbitrary input.
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The whole thing runs in a VM on [fly.io],
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so even if a sandbox escape is possible the worst an attacker can do is break the VM and ruin the playground for others.
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The whole backend even works on macOS (without the sandbox, _do not run in production!_),
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thanks to the [Darwin port of Hare](https://github.com/hshq/harelang) (and some of [my own fixes on top](https://github.com/hshq/harelang/compare/master...badboy:harelang:master)).
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I wrapped that in a [Nix flake](https://git.fnordig.de/jer/hare-nix) for easy installation.
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`nix develop` is all you need in the playground repository.
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The frontend is some basic HTML[^3], a couple of lines of JavaScript and [htmx].
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The code editor is [Codejar], a small one-file editor implementation.
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Code highlighting is handled by [highlight.js]
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No JavaScript transpiler or bundler needed.
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And yes, I use htmx, but still return JSON from the server and add it to the DOM using JavaScript in the right places.
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I'd rather not do the whole HTML escape in Hare right now server-side.
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Initially I built this using the JavaScript parts of [codapi],
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but I ended up rolling it myself using htmx,
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as that gives me more freedom placing & styling the button elements and controlling the output.
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## Production when?
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I will keep the playground running on Fly (for now).
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I do not plan to work on it much further.
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This is after all a playground for me to learn the language a bit and not a more serious long-term project.
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The code is open-source ([MPL licensed](https://git.fnordig.de/jer/hare-playground/src/branch/main/COPYING)).
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[playground]: https://hare-exec.fly.dev/
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[harelang]: https://harelang.org/
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[advances-on-c]: https://harelang.org/blog/2021-02-09-hare-advances-on-c/
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[hare-playground]: https://git.fnordig.de/jer/hare-playground
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[bubblewrap]: https://github.com/containers/bubblewrap
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[fly.io]: https://fly.io/
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[hare-nix]: https://git.fnordig.de/jer/hare-nix
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[htmx]: https://htmx.org/
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[codapi]: https://codapi.org/
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[hare-http]: https://git.sr.ht/~sircmpwn/hare-http
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[hare-json]: https://git.sr.ht/~sircmpwn/hare-json/
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[hare-logfmt]: https://git.sr.ht/~blainsmith/hare-logfmt
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[codejar]: https://medv.io/codejar/
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[highlight.js]: https://highlightjs.org/
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---
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_Footnotes:_
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[^1]: Sorry, no dark mode yet.
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[^2]: The syntax highlighter on this site doesn't know about hare, so this code is tagged as C.
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[^3]: Thanks to [Dennis](https://overengineer.dev/) for showing me how simple Flexbox is to use.
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