-- ++, concat, reverse from SOE

(++)        :: [a] -> [a] -> [a]  -- Note () around operator
[] ++ ys     = ys
(x:xs) ++ ys = x : (xs ++ ys)

concat    :: [[a]] -> [a]
concat xxs = foldr (++) [] xxs

-- First reverse:

reverse       ::  [a] -> [a]
reverse []     = []
reverse (x:xs) = reverse xs ++ [x]

-- Second reverse:

reverse xs = rev [] xs
  where rev acc []     = acc
        rev acc (x:xs) = rev (x:acc) xs
        
        
-- Third reverse:

revOp a b = b : a
reverse xs = foldl revOp [] xs