What uses have you found for higher-rank types in Haskell?

Question

Higher rank types look like great fun. From the Haskell wikibook comes this example:

foo :: (forall a. a -> a) -> (Char,Bool)
foo f = (f \'c\', f True)
         


        

Answer 1

Recently somebody asked a question here on Stack Overflow which could be solved with higher-rank types.

Another application is in the Scrap Your Boilerplate paper.