noRepetidos :: Eq a => [a] -> [a]

noRepetidos [3,2,5,2,4,7,3]  ==  [5,4,7]
noRepetidos "noRepetidos"    ==  "nRptids"
noRepetidos "Roma"           ==  "Roma"

import Test.QuickCheck

-- 1ª solución
-- ===========

noRepetidos :: Eq a => [a] -> [a]
noRepetidos xs =
  [x | x <- xs, ocurrencias x xs == 1]

-- (ocurrencias x ys) es el número de ocurrencias de x en ys. Por
-- ejemplo,
--    ocurrencias 3 [3,2,5,2,4,7,3]  ==  2
--    ocurrencias 5 [3,2,5,2,4,7,3]  ==  1
ocurrencias :: Eq a => a -> [a] -> Int
ocurrencias x ys =
  length [y | y <- ys, y == x]

-- 2ª solución
-- ===========
noRepetidos2 :: Eq a => [a] -> [a]
noRepetidos2 [] = []
noRepetidos2 (x:xs) | elem x xs = noRepetidos2 [y | y <- xs, y /= x]
                    | otherwise = x : noRepetidos2 xs

-- Equivalencia
-- ============

-- La propiedad de equivalencia es
prop_equivalencia :: [Int] -> Bool
prop_equivalencia xs =
  noRepetidos xs == noRepetidosA1 xs

-- La comprobación es
--    λ> quickCheck prop_equivalencia
--    +++ OK, passed 100 tests.