reader.ts

// === lib ===
type Contextualized<C, R> = (context: C) => R

const contexualize = contextConsumerGenerator => context => {
    const generator = contextConsumerGenerator()

    let contextConsumerIter = generator.next()
    let consume: any = contextConsumerIter.value

    while (!contextConsumerIter.done) {
        contextConsumerIter = generator.next(consume(context))
        consume = contextConsumerIter.value
    }
    
    return consume
}

const get = <Context extends { [key: string]: Value }, Value>(key: keyof Context) =>
    (context: Context): Value => context[key]

// ===========

type Context = {
    username: string,
    age: number
}


const fancyUserName = contexualize(
    function*() {
        const username = yield get('username')
        return `<<= ${username.toUpperCase()} =>>`
    }
)
// === can also be written like this ===
// -------------------------------------
//
// const fancyUserName: Contextualized<{ username: string }, string> =
//     ({ username }) => `<<= ${username.toUpperCase()} =>>`
//
// -------------------------------------


const friendsWithSameAgeRange = (margin: number) => contexualize(
    function*() {
        const users = [
            { username: 'boz', age: 12 },
            { username: 'biz', age: 13 },
            { username: 'buz', age: 14 },
        ]

        const currentUserAge = yield get('age')
        return users.filter(({ age }) => (age >= currentUserAge - margin && age <= currentUserAge + margin))
    }
)

const fancyNameWithFriends = contexualize(
    function* () {
        const funame = yield fancyUserName
        const friends = yield friendsWithSameAgeRange(1)

        return `${funame} :: ${friends.map(f => f.username)}`
    }
)

const context = { username: 'iboss', age: 12 }

// inject context at top level
console.log(fancyNameWithFriends(context)) // <<= IBOSS =>> :: boz,biz