Y-Combinator

y_combinator.clj

(defn factorial [n]
  (let [y (fn [f] (f f))
        f (fn [g] (fn [n] (if (zero? n) 1 (* n ((g g) (dec n))))))]
    ((y f) n)))

y_combinator.js

const y = (f) => f(f);
const f = (g) => (n) => (n === 0 ? 1 : (n * (g(g)(n - 1))));

export default (x) => y(f)(x);

y_combinator.rb

def factorial(n)
  y = -> (f) { f.(f) }
  f = -> (g) { -> (n) { n == 0 ? 1 : n * g.(g).(n - 1) } }
  y.(f).(n)
end