Implements the same diamond printing algorithm using the y-combinator (fix). Needs to be compiled with the -rectypes flag

diamond_overkill.re

open Core;

let fix = (f, g) => ((x, a) => f(x(x), a))((x, a) => f(x(x), a), g);

let make_line = (n, width) => {
  let blanks = String.make((width - n) / 2, ' ');
  let stars = String.make(n, '*');
  blanks ++ stars ++ blanks ++ "\n";
};

let make_rest = (width, step, combine, f, n) =>
  if (n <= 0) {
    "";
  } else {
    let rest = f(step(n));
    let this = make_line(n, width);
    combine(rest, this);
  };

let diamond = n => {
  let concat = (++);
  let step = n => n - 2;
  let above = make_rest(n, step, concat);
  let below = make_rest(n, step, Fn.flip(concat));
  let this = make_line(n, n);
  String.concat([fix(above, step(n)), this, fix(below, step(n))]);
};

let main = {
  let n = Sys.argv[1] |> int_of_string;
  printf("%s", diamond(n));
};