<p>Perhaps the best ending to an abstract I've read yet: <a href="http://arxiv.org/abs/1103.2841">"Multiplate only requires rank 3 polymorphism in addition to the normal type class mechanism of Haskell."</a>. (Oh, only rank-3 polymorphism, no big deal!)</p>
Use case for rank-3 (or higher) polymorphism?
<p>I've seen a few use cases for rank-2 polymorphism (the most prominent example being the <a href="http://www.haskell.org/haskellwiki/Monad/ST" rel="noreferrer">ST monad</a>), but none for a higher rank than that. Does anyone know of such a use case?</p>