new post: What's 2024 brought to Melange so far? (#174)

* new post: What's 2024 brought to Melange so far?

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Co-authored-by: Javier Chávarri <javier.chavarri@gmail.com>

* proof-reading review

* today's date

---------

Co-authored-by: Javier Chávarri <javier.chavarri@gmail.com>

blog/posts/whats-2024-brought-to-melange-so-far.md

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+---
+title: What's 2024 brought to Melange so far?
+date: 2024-04-09
+author: Antonio Monteiro
+gravatar: 45c2052f50f561b9dc2cae59c777aecd794f57269fa317f9c9c3365c2e00d16f
+twitter: '@_anmonteiro'
+---
+
+Last year we saw the [first major
+release](https://anmonteiro.substack.com/p/melange-10-is-here) of Melange. Over
+the course of 2023, what started as disjoint sets of tooling has gradually
+evolved into a cohesive development experience within the [OCaml
+Platform](https://ocaml.org/docs/platform).
+
+But we're not done yet. In the next few paragraphs, I'll tell you everything
+we've shipped so far in the first 3 months of 2024.
+
+
+---
+
+## Releasing [Melange 3](./announcing-melange-3)
+
+We released Melange 3.0 in February. This release mostly focused on addressing
+long-standing deprecations, crashes, error messages and making the Melange
+distribution leaner.
+
+One highlight of the Melange 3 release is the support for more versions of
+OCaml. Melange 3 supports OCaml 4.14 and 5.1; the next major release will
+additionally feature support for OCaml 5.2.
+
+[`reason-react`](https://github.com/reasonml/reason-react) and the Melange
+libraries in [`melange-community`](https://github.com/melange-community) were
+also updated and released with support for this new Melange major version.
+
+Melange 3 has been running in production at Ahrefs since its release. This is
+the largest Melange codebase that we are aware of (on the scale of tens of
+libraries with support for `(modes melange)` and `melange.emit` stanzas, across
+dozens of apps).
+
+
+## Emitting [ES6](https://github.com/melange-re/melange/issues/134)
+
+As the great majority of browsers [supports ECMAScript 2015
+(ES6)](https://caniuse.com/?search=es6.), we decided to bump the version that
+Melange targets. In
+[melange#1019](https://github.com/melange-re/melange/pull/1019) and
+[melange#1059](https://github.com/melange-re/melange/pull/1059) we changed the
+emission of `var` to `let` (and `const`, where possible). `let`'s lexical scope
+makes some closure allocations in `for` loops unnecessary, which we promptly
+removed in [melange#1020](https://github.com/melange-re/melange/pull/1020).
+This change also results in a slight reduction of bundle size as Melange emits
+a bit less code.
+
+Starting to emit ES6 also unblocks working on
+[melange#342](https://github.com/melange-re/melange/issues/342), which requests
+support for tagged [template
+literals](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals).
+We'll consider adding support for more features in ES6 and later on a
+case-per-case basis, if the added expressivity justifies the complexity of the
+implementation. Feel free to [open an
+issue](https://github.com/melange-re/melange/issues/new) if you feel that
+Melange should emit ES6 features that your project requires.
+
+
+## Identifying Melange exceptions in JavaScript
+
+Until Melange 3, exceptions originating from OCaml code compiled with Melange
+are roughly thrown as such:
+
+```js
+throw {
+  MEL_EXN_ID: "Assert_failure",
+  _1: ["x.ml", 42, 8],
+  Error: new Error()
+};
+```
+
+As stated in an [old ReScript
+issue](https://github.com/rescript-lang/rescript-compiler/issues/4506), the
+encoding above is at odds with user exception monitoring in popular vendors.
+
+We set out to fix this for Melange 4. The next release of Melange includes
+[melange#1036](https://github.com/melange-re/melange/pull/1036) and
+[melange#1043](https://github.com/melange-re/melange/pull/1043), where we
+changed the encoding to throw a dedicated `MelangeError` instance:
+
+```diff
+-throw {
++throw new Caml_js_exceptions.MelangeError("Assert_failure", {
+  MEL_EXN_ID: "Assert_failure",
+  _1: ["x.ml", 42, 8],
+- Error: new Error()
+-};
++});
+```
+
+Besides fixing the immediate issue – vendor SDKs for error monitoring now
+understand Melange runtime errors – this change brings a few additional
+benefits to users of Melange:
+
+- Detecting an exception originating from Melange-compiled code is now as easy
+  as using the JS `instanceof` operator to check if the exception is an
+  instance of `Caml_js_exceptions.MelangeError`.
+- `MelangeError` adds support for – and polyfills, if necessary – the
+  [`cause`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/cause)
+  property in instances of `Error`, which lets us squeeze out some extra
+  [browser support](https://caniuse.com/mdn-javascript_builtins_error_cause).
+  - Additionally, this enables even better integrations with 3rd party
+    monitoring tools, which will look for JavaScript error details in
+    `Error.prototype.cause`.
+
+
+## `melange.js` keeps getting better
+
+The [Melange 3
+announcement](./announcing-melange-3#runtime-stdlib)
+touched a bit on the improvements we did in the `Js.*` modules recently. We're
+always trying to improve the number of zero-cost bindings to functions in the
+JS runtime, and their quality. In next release, we're adding more functions to
+cover `Map`, `Set`, `WeakMap`, `WeakSet`, `BigInt` and `Iterator`.
+
+We're also taking advantage of Dune's obscure extension that models libraries
+like the OCaml `Stdlib` (`(using experimental_building_ocaml_compiler_with_dune
+0.1)` in `dune-project`, the `stdlib` field in `library` stanzas). Here's the
+difference between a "stdlib library" and a regular library: a stdlib library's
+main module depends on just a subset of the internal modules in the libraries,
+while others depend on this main module. Dune doesn't allow regular library
+modules to depend on their main module name.
+
+In the next release of Melange, we treat the `Js.*` modules like a "stdlib"
+library ([melange#1091](https://github.com/melange-re/melange/pull/1091)):
+modules are only accessible through the `Js.*` namespace; as a fortunate
+side-effect, we stop exposing `Js__Js_internal`, which could leak into some
+error messages, causing unnecessary confusion.
+
+
+## OCaml 5.2
+
+With OCaml 5.2 around the corner (the first
+[beta](https://github.com/ocaml/ocaml/archive/refs/tags/5.2.0-beta1.tar.gz)
+was released just a couple weeks ago), we made Melange ready for the upcoming
+release. In [melange#1074](https://github.com/melange-re/melange/pull/1074) and
+[melange#1078](https://github.com/melange-re/melange/pull/1078) we upgraded
+both the Melange core and the Stdlib to the changes in OCaml 5.2. As mentioned
+above, we're happy that the next release of Melange will support an even wider
+range of OCaml compiler versions, making 5.2 the latest addition to the
+supported compiler versions.
+
+## Leveraging Dune's potential
+
+### Making Dune faster
+
+Back in January, [Javi](https://twitter.com/javierwchavarri) found a
+performance regression in Dune
+([dune#9738](https://github.com/ocaml/dune/issues/9738)) after upgrading to
+Dune 3.13. The whole fact-finding process of profiling Dune's performance and
+working closely with the team to patch this regression
+([dune#9769](https://github.com/ocaml/dune/pull/9769)) ended up being quite the
+learning experience.
+
+Once the dust settled, Javi took the time to write a [blog
+post](https://tech.ahrefs.com/profiling-dune-builds-a8de589ec268) outlining
+some of the tools he used and the steps he used to gather information about
+Dune's runtime behavior.
+
+
+### Improving `dune describe pp`
+
+The command `dune describe pp` prints a given source file after preprocessing.
+This is useful to quickly inspect the code generate by a (set of) ppx.
+
+`dune describe pp` didn't, however, support [Dune
+dialects](https://dune.readthedocs.io/en/stable/overview.html#term-dialect). I
+[found out](https://github.com/ocaml/dune/issues/4470#issuecomment-1135120774)
+about this limitation when trying to get the preprocessed output of a ReasonML
+file.
+
+We recently set out to fix this problem. In
+[dune#10321](https://github.com/ocaml/dune/pull/10321) we made Reason files and
+dialects generally work within `dune describe pp`, and we followed up with the
+ability to print back the preprocessed output in the same dialect as the given
+file ([dune#10322](https://github.com/ocaml/dune/pull/10322),
+[dune#10339](https://github.com/ocaml/dune/pull/10339) and
+[dune#10340](https://github.com/ocaml/dune/pull/10340)).
+
+
+### [Virtual libraries](https://dune.readthedocs.io/en/stable/variants.html) in Melange
+
+From Dune's own documentation:
+
+> Virtual libraries correspond to Dune’s ability to compile parameterised
+libraries and delay the selection of concrete implementations until linking an
+executable.
+
+In the Melange case there's no executable linking going on, but we can still
+delay the selection of concrete implementations until JavaScript emission – in
+practice, this means programming against the interface of "virtual modules" in
+libraries and deferring the dependency on the concrete implementation until the
+`melange.emit` stanza.
+
+Or rather, this is _now_ possible after landing
+[melange#1067](https://github.com/melange-re/melange/pull/1067) and
+[dune#10051](https://github.com/ocaml/dune/pull/10051): in particular, while
+Dune support for Melange has shipped with virtual libraries since day one, it
+didn't support one of the most useful features that they provide: programming
+against the interface of a virtual module.
+
+
+### Melange rules work within the Dune sandbox
+
+Within the last month, we also fixed a bug where Dune didn't track all Melange
+dependencies precisely during the JavaScript emission phase. While the
+[originally reported issue](https://github.com/ocaml/dune/issues/9190) saw this
+bug manifest when moving modules across directories when `(include_subdirs ..)`
+is enabled, the fix we applied in
+[dune#10286](https://github.com/ocaml/dune/pull/10286) and
+[dune#10297](https://github.com/ocaml/dune/pull/10297) brings with it the
+fortunate side-effect of making Melange rules work in the [Dune
+sandbox](https://dune.readthedocs.io/en/stable/concepts/sandboxing.html).
+We're glad this issue is fixed since it could result in the [Dune
+Cache](https://dune.readthedocs.io/en/stable/caching.html) being poisoned,
+leading to very confusing results.
+
+To make sure that sandboxing keeps working with Melange, we enabled it by
+default in [dune#10312](https://github.com/ocaml/dune/pull/10312) for the
+Melange tests in Dune.
+
+
+## Towards Universal React in OCaml
+
+One of our goals for 2024 is to ship a good developer experience around
+"universal libraries" in OCaml, the ability to write a mixed OCaml / Melange
+codebase that shares most libraries and modules pertaining to DOM rendering
+logic.
+
+[Dave](https://twitter.com/davesnx) wrote
+[server-reason-react](https://github.com/ml-in-barcelona/server-reason-react)
+for this purpose. He also wrote a [post on his
+blog](https://sancho.dev/blog/server-side-rendering-react-in-ocaml) detailing
+the motivation behind this approach and what he wants to achieve with
+`server-reason-react`.
+
+While React component hydration in native OCaml is a challenge specific to
+Melange and React codebases, there are reusable primitives that we needed to
+implement in Dune to make it possible. They also unlock a host of new use cases
+for Dune that we expect will start getting adoption over time.
+
+Universal libraries are already deployed for a small subset of apps at Ahrefs.
+Javi wrote about how Ahrefs is [sharing component
+code](https://tech.ahrefs.com/building-react-server-components-in-ocaml-81c276713f19)
+for those apps.
+
+This past quarter, we took it a step further. We added support in Dune for
+libraries that share the same name, as long as they're defined in different
+build contexts ([dune#10222](https://github.com/ocaml/dune/issues/10222)).
+Support for libraries with the same name in multiple contexts landed in
+[dune#10307](https://github.com/ocaml/dune/pull/10307) and we'll be diving
+deeper into what led us to this solution and what it enables in a separate blog
+post.
+
+## Community at large
+
+### Bootstrapping Melange projects with `create-melange-app`
+
+In January, [Dillon](https://twitter.com/dillon_mulroy) released v1.0 of
+[`create-melange-app`](https://github.com/dmmulroy/create-melange-app), and
+we're now recommending it for bootstrapping [new Melange
+projects](https://melange.re/v3.0.0/getting-started.html#getting-started-automated-create-melange-app)
+
+`create-melange-app` is a friendly and familiar way to get started with OCaml,
+ReasonML, and Melange, geared towards JavaScript and TypeScript developers.
+
+`create-melange-app` quickly became a community favorite after its release,
+attracting a number of contributors that are helping make it better.
+
+### Melange Book
+
+[Feihong](https://twitter.com/feihonghsu) keeps making really good progress on
+our book [Melange for React Devs](https://react-book.melange.re), a
+project-based introduction to Melange for React developers. The newest chapter
+on [Cram testing](https://dune.readthedocs.io/en/stable/tests.html#cram-tests)
+guides you through the necessary steps for writing tests and snapshotting their
+output.
+
+There are more chapters in the pipeline that we'll be releasing incrementally
+as they're ready for consumption. We're also gathering feedback on the book; we
+invite you to go through it and open issues in the [GitHub
+repository](https://github.com/melange-re/melange-for-react-devs) for any
+material that deserves rewording.
+
+## Looking forward
+
+As we look forward, towards the next phase of Melange development, I'd like to
+take a moment to thank [Ahrefs](https://ahrefs.com), the [OCaml Software
+Foundation](https://ocaml-sf.org/) and my [GitHub
+sponsors](https://github.com/sponsors/anmonteiro/) for the funding and support,
+without which developing Melange wouldn't have been possible over this
+sustained period of time.
+
+On a personal note, it fills me with joy to have the opportunity to share the
+amazing work that Melange contributors have been putting in. It represents a
+stark contrast from not long ago and the Melange project and community are
+better for it. Thank you everyone!
+
+As a final note, we thank you for reading through our updates for the first
+three months of 2024! As we finish planning our work for the next period, we'll
+share updates on what's to come for Melange in the not-so-distant future.
+
+Until then, stay tuned and happy hacking!
+