Future

future.ts

// simple
const future = f =>  ({
        fork: g => f(g),
        map: g => future(k => f(l => k(g(l)))),
        flatMap: g => future(k => f(l => g(l).apply(k)))
    })

const getHttp = callback => {
    const id = Math.floor(Math.random() * 5000 + 1000)
    setTimeout(() => callback(`hej ${id}`), id) 
}

const mapping = (s:string) => `MAPPING(${s})`
const midway = future(getHttp).map(mapping)
const final = midway.map(x => `IGEN(${x})`)

// With reject

interface IFuture<T, U> {
   of(U): IFuture<any, U>;
   reject(T) : IFuture<T, any>;
   map<V>(a:(U) => V): IFuture<T, V>;
   fork<T, U>(a:(T) => void, b:(U) => void) : void;
}

const future2 = <T, U>(f:(a:(T) => void, b:(U) => void) => void) => ({
    fork: (rej:(a:(T) => void) => void, res:(b:(U) => void) => void) => f(rej, res),
    of: (x:U) => future2((_, res) => res(x)),
    reject: (x:T) => future2((rej, _) => rej(x)),
    map: <V>(g:(a:U) => V) : IFuture<T, V> => future2((rej, res) => f(a => rej(a), b => res(g(b))))
})

const getHttp2 = (rej:(string) => void, res: (string) => void) => {
    const id = Math.floor(Math.random() * 1000 + 1000)
    setTimeout(() => (id % 2 == 0) ? 
                        rej(`det gick åt skogen efter  ${id} ms`) : 
                        res(`det gick bra efter ${id} ms`) , id) 
}


const withreject : IFuture<string, string> = future2(getHttp2).map(x => x + 'xxx').map(y => 'yyy' + y)
withreject.fork(x => console.error('error: ' + x), x => console.log('success: ' + x))