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