Co-inductive types in Elm

A way to implement "virtual class" in functional programming languages.

The core idea is defining the following type:

type Component
    = Unroll { update : () -> Component, render : () -> String }

Component Example

In a game engine, you may have different kinds of components, each of them has its own datatype and update/render functions.

This repo tries to implement a simple component updater in Elm that updates different components.

Our goals are:

Different components may use their own datatype (NOT exposed to our updater)
We can design Component datatype for users but the type must not change when users are writing a component
Component has a data of type a, an update function of type a -> a, and a render function of type a -> String

We may want to add messages to let components react to different events.

However, a component may want to decide the message types by themselves (besides world events).

Please read the example to understand how this works.

Remember these three files belong to diffrent modules (may be in different libraries).

Comp1: A component that can increase or decrease its own value. Note that Comp1 is independent of Main
Comp: A library to manage components, provided for both component designers and users
Main: An end user who uses Comp1