Y Combinator http://r6.ca/blog/20060919T084800Z.html create by https://stackoverflow.com/users/9859/rampion

y-combinator

-- The Y combinator, discovered by Haskell B. Curry, is defined as: Y = \f.(\x.f (x x)) (\x. f (x x))
-- haskell

newtype Mu a = Mu (Mu a -> a)
y f = (\h -> h $ Mu h) (\x -> f . (\(Mu g) -> g) x $ x)


-- javascript
const factSource = partial => n => n === 0 ? 1 : n * partial(n - 1);

// Y Combinator: Y = \f. (\x. f (x x)) (\x. f (x x))

y = f => ( x => n => f(x(x))(n)   )(
           x => n => f(x(x))(n)   );

const fac = y(factSource);

// OR

const Y = f => (x => x(x))(x => f(y => x(x)(y)));

const factorial = f => (x => (x === 1 ? 1 : x * f(x - 1)));