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JSX

Note: If you are looking for ReasonReact specific JSX infos, please refer to the ReScript JSX docs instead.

Voudriez-vous un peu de syntaxe HTML dans votre Reason ? Si non, sautez vite cette section et faites semblant de n'avoir rien vu !

Reason supporte la syntaxe JSX, avec quelques légères différences par rapport à celle utilisée dans ReactJS. Le JSX de Reason n'est pas lié à ReactJS ; il se traduit en classiques appels de fonctions :

Balise avec une majuscule

<MyComponent foo={bar} />

becomes

([@JSX] MyComponent.createElement(~foo=bar, ~children=[], ()));

Balise sans majuscule

<div foo={bar}> child1 child2 </div>;

devient

([@JSX] div(~foo=bar, ~children=[child1, child2], ()));

Fragment

<> child1 child2 </>;

devient

([@JSX] [child1, child2]);

Enfants

<MyComponent> foo bar </MyComponent>

This is the syntax for passing a list of two items, foo and bar, to the children position. It desugars to a list containing foo and bar:

([@JSX] MyComponent.createElement(~children=[foo, bar], ()));

So naturally, <MyComponent> foo </MyComponent> desugars to ([@JSX] MyComponent.createElement(~children=[foo], ())). I.e. whatever you do, the arguments passed to the children position will be wrapped in a list. What if you don't want that? What if you want to directly pass foo without an extra wrapping?

Spread d'enfants

To solve the above problem, we've introduced

<MyComponent> ...foo </MyComponent>

This passes the value foo without wrapping it in a list. Aka, this desugars to:

([@JSX] MyComponent.createElement(~children=foo, ()));

This is extra useful in the cases where you are handled foo that is already a list of things, and want to forward that without wrapping it an extra time (which would be a type error). It also allows you to pass arbitrary data structures at children position (remember, JSX children is really just a totally normal prop):

<MyComponent> ...((theClassName) => <div className=theClassName />) </MyComponent>;
<MyForm> ...("Hello", "Submit") </MyForm>;

Utilisation

Here's a JSX tag that shows most of the features.

<MyComponent
  booleanAttribute={true}
  stringAttribute="string"
  intAttribute=1
  forcedOptional=?{Some("hello")}
  onClick={send(handleClick)}>
  <div> {"hello"} </div>
</MyComponent>

Différences avec le JSX de JavaScript

  • Les attributs et enfants ne nécessitent pas l’utilisation de {}, mais nous les montrons tout de même pour faciliter l’apprentissage. Une fois que vous refmttez votre fichier, certains d’entre deux s’en vont et d’autres deviennent des parenthèses.
  • Le prop spread de JSX n’est pas supporté : <Foo {...bar}>. Encore que dans le même ordre d'idées, nous avons le children spread, présenté plus haut : <Foo> ...baz </Foo>.
  • Le punning !

Punning

"Punning" refers to the syntax shorthand for when a label and a value are the same. For example, in JavaScript, instead of doing return {name: name}, you can do return {name}.

Reason JSX supports punning. <input checked /> is just a shorthand for <input checked=checked />. The formatter will help you format to the latter whenever possible. This is convenient in the cases where there are lots of props to pass down:

<MyComponent isLoading text onClick />

Consequently, a Reason JSX component can cram in a few more props before reaching for extra libraries solutions that avoids props passing.

Note that this is a departure from ReactJS JSX, which does not have punning. ReactJS' <input checked /> desugars to <input checked=true />, in order to conform to DOM's idioms and for backward compatibility.

Frameworks using JSX

  • ReveryUI: A ReactJS-like UI framework for building cross-platform GUI applications

Tip & Tricks

For library authors wanting to take advantage of the JSX: the [@JSX] attribute above is a hook for potential ppx macros to spot a function wanting to format as JSX. Once you spot the function, you can turn it into any other expression.

This way, everyone gets to benefit the JSX syntax without needing to opt into a specific library using it.

JSX calls supports the features of labeled functions: optional, explicitly passed optional and optional with default.

Design Decisions

The way we designed this JSX is related to how we'd like to help the language evolve. See the section "What's the point?" in this blog post.

The ability to have macros in the language + the library-agnostic JSX syntax allows every library to potentially have JSX without hassle. This way, we add some visual familiarities to the underlying OCaml language without compromising on its semantics (aka how it executes). One big goal of Reason is to let more folks take advantage of the beautiful language that is OCaml, while discarding the time-consuming debates around syntax and formatting.

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