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

It may be that you have encountered problems where higher ranked types would be useful, but failed to realise it. For instance in the Darcs example they could easily have implemented it without higher ranked types. Instead there would have been preconditions on some functions that the caller would have to make sure they comply with, such as choosing the right instantiation of a function for the repository format.

The advantage of the higher ranked type is that it transfers responsibility for getting this right from the programmer to the compiler. With the conventional approach, if a Darcs developer made a mistake with the repository type the result would either be a run-time error or else corrupted data. With the higher ranked type the developer gets a type error at compile time.