callbag-typescript-proof-of-concept

readme.ts

/**
 * Callbag loves TypeScript
 * 
 * Copy-paste this into http://www.typescriptlang.org/play/index.html
 */

// A Callbag dynamically receives input of type I 
// and dynamically delivers output of type O 
type Callbag<I, O> = {
  (t: 0, d: Callbag<O, I>): void;
  (t: 1, d: I): void;
  (t: 2, d?: any): void;
};

// A source only delivers data
type Source<T> = Callbag<void, T>;

// A sink only receives data
type Sink<T> = Callbag<T, void>;

// An operator is a function Source<A> => Source<B>
const map = <A, B>(f: (x: A) => B) => (inputSource: Source<A>) => {
  const outputSource: Source<B> = (start, sink) => {
    if (start !== 0) return;
    inputSource(0, (t, d) => {
      sink(t, t === 1 ? f(d) : d);
    });
  };
  return outputSource;
};

// Here is an example source
const numberListenable: Source<number> = (t: 0 | 1 | 2, d) => {
  if (t !== 0) return;
  const sink: Callbag<number, void> = d;
  let i = 0;
  setInterval(() => {
    sink(1, i++);
  }, 1000);
};

const stringListenable = map((x: number) => `${x}`)(numberListenable);

// Example sink
const numberSink: Sink<number> = (t: 0 | 1 | 2, d) => {
  if (t === 1) {
    console.log(d);
  }
};

const stringSink: Sink<string> = (t: 0 | 1 | 2, d) => {
  if (t === 1) {
    console.log(d.toUpperCase());
  }
};

// This works:
numberListenable(0, numberSink);

// This doesn't:
numberListenable(0, stringSink);

// This works:
stringListenable(0, stringSink);