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
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