Re: [Haskell-cafe] Question on rank-N polymorphism

The idea is that fs accepts a polymorphic function as its argument. What type signature can I specify for f in order to compile this code?

As you said yourself, you need to add a type signature to fs:

{-# LANGUAGE RankNTypes #-}

fs :: ((forall a . ((a, a) -> a)) -> t) -> (t, t) fs g = (g fst, g snd)

examples = (fs id, fs repeat, fs (\x -> [x]), fs ((,)id))

Hope this helps, Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.