Custom Types

[lsjroberts]

Custom types are also known as "union types", "tagged unions" and "algebraic data types".

While typescript provides a way to have compile-time union types, they are no longer available at runtime, which means they can't be used for active behaviours (such as messages in wool/browser).

Basic usage

import { Type } from 'wool/core';

const Increment = Type.custom('Increment');
const Decrement = Type.custom('Decrement');

let value = 0;

const msg = Increment();

Type.matchOn(msg, {
  [Increment]() { value = value + 1; },
  [Decrement]() { value = value - 1; },
});

console.log(value);
// 1

Parameters

import { Type } from 'wool/core';

const SetValue = Type.custom('SetValue', Type.int);

const msg = SetValue(42);

Type.matchOn(msg, {
  // ...
  [SetValue](newValue) { value = newValue; },
});

console.log(value);
// 42

import { String, Type } from 'wool/core';

const SetGreeting = Type.custom('SetGreeting', Type.string, Type.int)

let value = 'Hi';

const msg = SetGreeting('Hello', 3);

Type.matchOn(msg, {
  [SetGreeting](greeting, times) {
    value = String.repeat(greeting, times);
  },
});

console.log(value);
// "HelloHelloHello"

[lsjroberts]

A better alternative:

const Msg = Type.custom({
  Increment: [],
  SetValue: [Type.int],
  SetGreeting: [Type.string, Type.int],
});

const msg = Msg.Increment();
// Msg.SetValue(42);
// Msg.SetGreeting('Hello', 3);

Msg.match(msg, {
  Increment() {
    value = value + 1;
  },
  SetValue(newValue) {
    value = newValue;
  },
  SetGreeting(greeting, times) {
    value = String.repeat(greeting, times);
  },
});

Msg.match would throw an error if not all the names were accounted for.

[lsjroberts]

Though it should be feasible to have a default match as well:

Msg.match(msg, {
  Increment() {
    value = value + 1;
  },
  _() {
    // do some default action
  },
});

[lsjroberts]

And it should return the returned value:

value = Msg.match({
  Increment: () => value + 1,
  // ...
})

[lsjroberts]

Is Msg.case a better name?