Purity leads to compositionality.

  if(player.score > 0)
    player.setSwizzle(7);
  else
    player.setSwizzle(player.swizzle + 1);

Analyse this program. It intertwines purity with impurity (setSwizzle).

  S s = player.score > 0 ? 7 : player.swizzle + 1;
  player.setSwizzle(s);

Now we have untangled it a little.