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Using Gleam in HTML with as little JavaScript knowledge as possible
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:slug: using-gleam-in-html-low-js
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:date: 2024-09-09 16:28
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:tags: gleam, software, web
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:category: programming
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:keywords: javascript module, gleam, frontend
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:lang: en
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:translation: false
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:status: published
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I've been looking at the `Gleam language <https://gleam.run>`__ recently. Among other features, it can be compiled to JavaScript, and thus presumably used on web frontend. I wanted to try that. This is a short tutorial on how to do that with little idea how JavaScript is supposed to work.
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My initial JS knowledge: ``alert(3)``, ``onclick`` and some basic selectors, i.e. the little stuff that is useful to add minor interactivity to HTML pages and implement trivial ViolentMonkey scripts. Namely: I have no knowledge about modules and whatnot, and this post is just a result of my trial-and-error attempt at embedding my Gleam. I succeeded, but still have no idea what I am doing :-)
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My code
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=======
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Let's start with some trivial code, in ``src/lol.gleam`` (in a project initialised with ``gleam new lol`` and the ``repeatedly`` package added with ``gleam add repeatedly``)::
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import gleam/io
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import repeatedly
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pub fn say_hello() {
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repeatedly.call(2000, Nil, fn(_state, _call) {io.println(get_greeting())})
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}
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pub fn get_greeting() {
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"Hello World!"
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}
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pub fn main() {
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io.println("3, 2, 1, go!")
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}
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This snippet has many of the basic stuff I might need in future: it does some io, it uses another package [#stdlib-package]_, it returns data I will want to show. At first I am mostly interested in multiplatform stuff (i.e. running also on the BEAM backend), so I don't want to use any DOM frameworks at first, though I will mention some of the ways later in the post.
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So, build this for JavaScript: ``gleam build --target javascript``, and *stuff happens*. It is not very clear what to do now, but ``grep``ping through the ``build/`` directory shows that the built code lives in ``build/dev/javascript/lol/lol.mjs``.
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Embedding the code
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==================
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Here comes the fun part: the code is in JavaScript *module*, not plain code. That comes with several surprises:
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- It can only be imported from other modules, not by “plain” code in global scope
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- Due to CORS, imports only work with network schemes, not with the ``file://`` one
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- Due to scoping, I have not found a way of using the functions from devtools console
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Big wins for the platform /s, but we have to live with that, so let's try to write the respective HTML for this. I put that in ``main.html`` in the project root, but it does not probably matter. The code::
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<!doctype html>
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<html>
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<head>
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<meta charset=utf-8> <!-- firefox complains otherwise -->
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<title>Buh</title>
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</head>
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<body>
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<!-- some random elements to work with -->
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<button id=butt>Klik mee</button>
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<p id=uwu>
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<!-- the “binding” to our Gleam code. This probably needs to be at the end of the page, since it needs to be able to use the selectors. -->
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<script type=module> //It has to be a module to allow imports
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import * as lol from "./build/dev/javascript/lol/lol.mjs";
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// Set the text of one element to the computed stuff:
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let par = document.getElementById('uwu');
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par.innerText = lol.get_greeting();
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// Let's have an interactive button (I love the come-from pattern, but using `onclick` would be even trickier…)
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let butt = document.getElementById('butt');
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butt.addEventListener('click', lol.say_hello);
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// Apparently, the `main` function does not get run automatically, so call that explicitly.
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lol.main();
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</script>
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</body>
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</html>
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Shoutout to `the person who asked the same question on StackOverflow <https://stackoverflow.com/questions/53630310/use-functions-defined-in-es6-module-directly-in-html>`__. From this point on, we only need a HTTP server; luckily, Python has one in stdlib, so just calling ``python3 -m http.server 12312`` in the project root lets us load ``http://localhost:12312/main.html`` and see our page.
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Next steps, alternatives et cetera
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==================================
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When developing web frontend in Gleam, the more common way is using a framework like `Lustre <https://hexdocs.pm/lustre/index.html>`__ or at least a DOM library (there are `several <https://packages.gleam.run/?search=dom>`__, I have no idea how mature those are). I did not go this route yet, because I am more interested in using backend-agnostic Gleam code from JavaScript and don't mind writing the trivial bindings in JavaScript. [#netzpevnik-gleam]_
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Also, a common thing to do is using a minifier+bundler like `esgleam <https://hexdocs.pm/esgleam/index.html>`__ (it uses `esbuild <https://esbuild.github.io/>`__ under the hood), so that the whole project is in one file. I don't think I need that now (having the JS be readable is more important to me atm and I don't want to complicate things further), but there is at least `one tutorial <https://erikarow.land/notes/esgleam-embed>`__ on how to do that.
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Also, during writing of this article I realised Gleam can run all the JS code from itself, so I could have a ``js_main`` function that would bind the HTML from Gleam itself. But this is probably more readable and separated anyway.
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-------
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.. [#stdlib-package] Well, as far as I understand it, Gleam's stdlib is actually just another package anyway.
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.. [#netzpevnik-gleam] I will probably use some kind of framework eventually as a part of `netzpevnik <https://gitea.ledoian.cz/LEdoian/netzpevnik>`__, but I am exploring the technologies that will be involved, so I want to keep stuff simple.
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