iguales :: [a] -> [a] -> [a]
iguales a b = compareHeads a b
iguales (a:as) (b:bs) = compareHeads a b ++ iguales as bs

compareHeads :: (Num a) => a -> a -> a
compareHeads x y = if (x == y) then x else 0

Ejercicios Relacion 2.hs:89:38:
    Couldn't match type `a' with `[a]'
      `a' is a rigid type variable bound by
          the type signature for iguales :: [a] -> [a] -> [a]
          at Ejercicios Relacion 2.hs:88:1
    In the first argument of `compareHeads', namely `a'
    In the first argument of `(++)', namely `compareHeads a b'
    In the expression: compareHeads a b ++ iguales as bs

Ejercicios Relacion 2.hs:89:40:
    Couldn't match type `a' with `[a]'
      `a' is a rigid type variable bound by
          the type signature for iguales :: [a] -> [a] -> [a]
          at Ejercicios Relacion 2.hs:88:1
    In the second argument of `compareHeads', namely `b'
    In the first argument of `(++)', namely `compareHeads a b'
    In the expression: compareHeads a b ++ iguales as bs