YCombinator

fib-y-comb.js

const genFib = f => n => {
  if (n <= 1) return 1;
  return f(n - 1) + f(n - 2);
};

const fib = someFunc(genFib);
/*
 * Above fib is the result we want. But we don't know HOW to implement someFunc
 */
// From Line 6, we know someFunc is a function that will return fib, with a parameter genFib passed in
// So, a naive implementation of someFunc would be the following (let's assume "fib" is defined at this point)
const fib = (f => f(fib))(genLib);
//
const fib = (f => genLib(n => fib(n)))(genLib); // substitute f inside the function with its real value: genLib
//
const fib = (f => genLib(n => someFunc(genLib)(n)))(genLib); // we don't have fib before defining this line, so substitute fib as well
// We get:
const someFunc = f => genLib(n => someFunc(genLib)(n)); // we now have a version of someFunc that don't rely on fib to be defined
//
const someFunc = f => f(n => someFunc(f)(n)); // genLib is f, so someFunc don't rely on genLib as well
// Then,
const yComb = f => f(n => yComb(f)(n));