Modeling of parametrized Messages and a Generic Dispatcher

MessageDispatcher.ts

// ============================================================
// Sample code:
// Modeling of parametrized Messages and a Generic Dispatcher
//
// Author: @fvilante
// ============================================================

// Messages (or events, or actions...)

interface Increment {
    kind: 'Increment',
    toSub: number,
    foo: undefined
}
const Increment = (toSub: number): Increment =>
    ({ kind: 'Increment', toSub, foo: undefined })


interface Decrement {
    kind: 'Decrement',
    toAdd: number
};
const Decrement = (toAdd: number): Decrement =>
    ({ kind: 'Decrement', toAdd })


type Message =
    | Increment
    | Decrement


// Model 

interface Model {
  count: number
}

const initialModel: Model = {
    count: 0
}

// Update 

type Update = (_: Model) => (_: Message) => Model

const update: Update = model => msg => { 
    switch (msg.kind) {
        case 'Decrement':
            return {
                ...model,
                count: model.count - msg.toAdd
            }
        case 'Increment':
           return {
               ...model,
                count: model.count + msg.toSub
            }
      default:
        const exhaustiveCheck: never = msg
    }
}

// reducer

type Reduce = (_:Update) => (_:Model) => (_:Message[]) => Model
const reduce: Reduce = update => model => msgs => {
    let m = model
    for (let i of msgs) {
        m = update(m)(i)
    }
    const r = m
    return r
}

// use

const main = () => {  

  const msgs = [
      Increment(10),
      Increment(5),
      Decrement(40),
    ]

  console.log('Before', initialModel) // Before { count: 0 }
  
  const m = reduce(update)(initialModel)(msgs)
  
  console.log('After', m) // After { count: -25 }
}


// execution
main()