Pure TypeScript

io.ts

// Did you know JavaScript has pure functions*?
// *: just add asterisks!

/**
 * Describes a computation that requires `E` to produce an `A`. Has to be run
 * to do anything.
 * 
 * We could also do something like:
 * `type Async<E,A> = Generator<void,Promise<A>,E>`
 */
type IO<E,A> = Generator<void,A,E>;

/**
 * The random effect 🎰
 */
interface Random {
    random(): number
}

/**
 * The logging effect 📝
 */
interface Log {
    log(msg: any): void
}

function* random(): IO<Random, number> {
    console.log('running random');
    // `yield` here fetches the `E`
    return (yield).random();
}

function* log(msg: any): IO<Log, void> {
    console.log('running log');
    (yield).log(msg);
}

// Look Haskell, I'm pure! λ
{
    random(); 
    log('complete silence 😶')
}

function unsafeRun<E,A>(io: IO<E,A>, e: E): A {
    let res;
    do {
        // Ignoring errors 🚨
        res = io.next(e)
    } while (!res.done)
    return res.value as A;
}

function* main() {
    // We can use `yield*` like `<-` in Scala and Haskell
    yield* log("i'm going to log something random");
    let rn = yield* random();
    yield* log(rn);
}

main() // Still pure 🚔

// TypeScript somehow magically infers `E` to be `Log & Random`
// Here we could pass in mocks for testing 🧪
unsafeRun(main(), {
    log: (msg) => console.log(msg),
    random: () => Math.random(),
})

I think I can definitely live with those yield*. Better than fp-ts pipe's!

@thoradam Did you continue any on this stuff? I'm still intrigued :) I just revisited fp-ts and io-ts and noticed he had somewhat better do-notation that I didn't find last time I looked:

https://gcanti.github.io/fp-ts/guides/do-notation.html

But there's something really cool here when stuck in ts…

@hedefalk No I didn’t take this further, not writing any TS anymore ^_^ Another thing I looked into at the time was overloading async/await, but unfortunately TS fixes it to Promise. I had looked at the fp-ts do notation but I think it’s way too noisy :S