2Nov2015 ejercicio4

{- Ejercicio 4
    Libreria generica
    -}
    data Tipo = Musica | Libros deriving Show
    data Producto = Prod Tipo Float

    class Libreria a where
        damePB :: a -> Float
        dameIVA :: a -> Float
        dameTasa :: a -> Float

    instance Eq Tipo where
        t1 == t2 = (show t1) == (show t2)

    instance Eq Producto where
        (Prod t1 x1) == (Prod t2 x2) = (x1 == x2) && (t1 == t2)

    instance Libreria Producto where
        damePB (Prod _ x) = x
        dameIVA (Prod Musica x) = 0.21
        dameIVA (Prod Libros x) = 0.15
        dameTasa (Prod a x) = x * dameIVA (Prod a x)

    pvp :: Producto -> Float
    pvp (Prod a x) = x + dameTasa (Prod a x)