const head = xs => xs[0];
const tail = xs => xs.slice(1);
const add = a => b => a + b;

const dot = Y(f => n => n < 1 ? '' : '.' + f(n - 1));
const factorial = Y(f => n => n < 1 ? 1 : n * f(n - 1));
const reduce = f => Y(g => y => xs => xs.length < 1 ? y : g(f(y)(head(xs)))(tail(xs)));

dot(3) //> "..."
factorial(4) //> 24
reduce(add)(0)([1, 2, 3]) //> 6