title: 'Repa' performance for planetary simulation
body: <p>I have written a simulation of the outer planets of the solar system using the <a href="https://en.wikipedia.org/wiki/Semi-implicit_Euler_method" rel="noreferrer">Euler symplectic method</a> and implemented this a) using <code>repa</code> and b) using <code>yarr</code>.</p>
  
  <p><a href="http://idontgetoutmuch.wordpress.com/2013/08/06/planetary-simulation-with-excursions-in-symplectic-manifolds-6/" rel="noreferrer"><code>yarr</code> seems to perform about x30 quicker than <code>repa</code></a>.</p>
  
  <p>Given this, I didn't even try to use parallelism. Is there any obvious performance problems in my <code>repa</code> code? The repository is at <a href="https://github.com/idontgetoutmuch/Leapfrog" rel="noreferrer">github</a>. I can produce a cut-down <code>repa</code>-only version if this is helpful, but then you won't get the performance comparison against <code>yarr</code>.</p>
  
  <p>Alternatively, how do I debug performance issues in <code>repa</code>?</p>