[Haskell-beginners] Explanation of composite with foldl

I see this on the Haskell 99 questions https://wiki.haskell.org/99_questions/Solutions/2 which will return the second-from-last element of a list lastbut1 :: Foldable f => f a -> a lastbut1 = fst . foldl (\(a,b) x -> (b,x)) (err1,err2) where err1 = error "lastbut1: Empty list" err2 = error "lastbut1: Singleton" I understand how the code works, but not the significance of the type declaration. That looks like a type class. It works without it, I believe. Why have we used Foldable type class? Likewise with this lastbut1safe :: Foldable f => f a -> Maybe a lastbut1safe = fst . foldl (\(a,b) x -> (b,Just x)) (Nothing,Nothing) What's happening with the type definition? LB