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webbureaucrat

from legacy to functional and very little in between.

ReasonML Journey Part II: Subtyping and ExtendableEvent

The first stop on my quest to wrap a JavaScript library in ReasonML is a small, simple JavaScript interface ExtendableEvent, a class with no properties and only one method on top of what it inherits from Event. In this brief blog post I will set out a simple template for subtyping in ReasonML.

Source as a learning resource

There are not many BuckleScript tutorials out there, so in addition to the official documents, I've found that one of the best ways to learn ReasonML is by reading the ReasonML community libraries. The bs-webapi-incubator, for example, includes a helpful explanation of subtyping and inheritance in ReasonML. It gives the following example:

type _element('a);
type element_like('a) = node_like(_element('a));
type element = element_like(_baseClass);

Candidly, while I think I pretty much understand what's going on here, I do not understand it well enough to explain it, so that will be left as an exercise for the user or for a future blog post after I get my bearings. For now, I'm satisfied to know that this code makes a type element that's a subtype of node, or rather, it's close cousin node_like which takes a type parameter.

This code snippet is an example of source code from the standard library itself, not a project that references the standard library, so to make this code compile, we need to reference the Dom module in the standard library like so:

type _element('a);
type element_like('a) = Dom.node_like(_element('a));
type element = element_like(Dom._baseClass);

This code snippent will compile, and we're now well on our way to recreating ReasonML's element type.

But that's not what we're here for. We're here to write create ExtendableEvent from Event, and even though I only kind of understand this code, I can pretty copy this format for my use case:

type _extendableEvent('a);
type extendableEvent_like('a) = Dom.event_like(_extendableEvent('a));
type extendableEvent = extendableEvent_like(Dom._baseClass);

Now all I have to do is paste this into a .re file in src/ and run npm run build, and I have the first step done. You'll have to waitUntil the next blog post to see how to extend this subtype to include a new method.

I write to learn, so I welcome your constructive criticism. Report issues on GitLab.

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