Compare lengths of finite and infinite lists

Len.hs

data Length = forall x. Length { unLength :: [x] }

instance Eq Length where
  (==) (Length []) (Length []) = True
  (==) (Length (_:xs)) (Length (_:ys)) = Length xs == Length ys
  (==) _ _ = False

instance Ord Length where
  compare (Length []) (Length []) = EQ
  compare (Length (_:_)) (Length []) = GT
  compare (Length []) (Length (_:_)) = LT
  compare (Length (_:xs)) (Length (_:ys)) = compare (Length xs) (Length ys)

toLength :: Length -> Int
toLength (Length xs) = length xs

length' :: [x] -> Length
length' = Length