title: Catalan numbers in functional languages
body: <p>The Catalan numbers satisfy the recurrence </p>
  
  <p><img src="https://upload.wikimedia.org/math/e/7/4/e747709abf456adcec2cce0cc95e96a4.png" alt="Catalan Recurrence"></p>
  
  <p>Of course Catalan numbers have a closed form expression involving binomial coefficients. Also we can express C_n in terms of C_{n-1} alone. What I am wondering is how one can implement this kind of convolution in functional languages like SML or Haskell.</p>