dia :: Int -> Int -> Int -> String

dia 21 12 2016  ==  "Miercoles"
dia 14  4 1936  ==  "Martes"

-- 1ª solución
-- ===========

-- Comentario: usar que el 1-1-2007 cayó en lunes

dSem :: [(String,Int)]
dSem = [("Lunes",0),("Martes",1),("Miercoles",2),("Jueves",3),
        ("Viernes",4),("Sabado",5),("Domingo",6)]

dMes :: [(Int,Int)]
dMes = [(1,31),(2,28),(3,31),( 4,30),( 5,31),( 6,30),
        (7,31),(8,31),(9,30),(10,31),(11,30),(12,31)]

busca :: a -> [(a1,a2)] -> (a -> a1 -> Bool) -> [(a1,a2)]
busca x xs p  =  [(x',y) | (x',y) <- xs, p x x']

diaAno :: (Int,Int,Int) -> Int
diaAno (d,m,a)  =  d + sum [y | (x,y) <- busca m dMes (>)]

dia :: Int -> Int -> Int -> String
dia d m a = head [fst (x,y') | (x,y') <- dSem, n == y']
  where  n = mod ((diaAno (d,m,a) + (a - 2007)*365) + x) 7
         x | d <= 29 && m <= 2  =  div (a - 2007) 4 - 1
           | otherwise          =  div (a - 2007) 4


-- 2ª solución
-- ===========

-- Usando la congruencia de Zeller http://bit.ly/2gYxHym

dia2 :: Int -> Int -> Int -> String
dia2 d m a = diasSemana !! h
  where m1 = if   (m < 3)
             then m + 12
             else m

        a1 = if   (m1 > 12)
             then a - 1
             else a

        k  = a1 `mod` 100
        j  = a1 `div` 100

        h  = ( d
             + ((13 * (m1 + 1)) `div` 5)
             + k
             + (k `div` 4)
             + (j `div` 4)
             - (2 * j)
             ) `mod` 7

diasSemana :: [String]
diasSemana = [ "Sabado"
             , "Domingo"
             , "Lunes"
             , "Martes"
             , "Miercoles"
             , "Jueves"
             , "Viernes"]