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)

Now we can evaluate foo id without the compiler exploding. This example is quickly followed in the book by the real-world example I have seen in a few other places: the ST monad and runST. That's pretty cool.

But I have yet to come across a situation where I solve a problem by writing my own function with an argument of higher-rank. Have you? What examples do you have of rank-2 or rank-n polymorphism in the wild?

Answer 1

Take a look at functions like withRepoLock in the Darcs source.

Darcs has support for multiple repository formats, and that support is expressed via a typeclass. So you can write code that is generic over repository formats. When actually reading an on-disk repository you want to dispatch to that code through some common code that figures out what format the repository is in and chooses the right typeclass instantiation.