[Haskell-cafe] forall & ST monad

I'm having trouble understanding the explanation of the meaning of the signature of runST at http://en.wikibooks.org/wiki/Haskell/Existentially_quantified_types I could try to read the article a couple of times again, but are there any other good readings about these existentially quantified types and how the ST monad works? I'm currently only using forall in combination with type classes, as in data MVC m v = forall c. Controller c => MVC m v c However - as the article mentions - forall can also be used to constrain the scope of type variables, but I don't get it yet. Thanks!