Prints diamonds to the screen

diamond.re

open Core;

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

let rec make_rest = (n, width, concat) =>
  if (n <= 0) {
    "";
  } else {
    let rest = make_rest(n - 2, width, concat);
    let this = make_line(n, width);
    concat(rest, this);
  };

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

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