Am Donnerstag, 23. Oktober 2008 08:07 schrieb Benjamin L.Russell:
How is it possible to define a user datatype consisting of instances of String?
Suppose I am preparing a party for wine connoisseurs, and want to define the following data types:
data Wine = Red | White data Red = "Merlot" data White = "Sauvignon Blanc" data Entree = "pork" | "chicken" | "tuna" data SideDish = "garlic bread" | "mozzarella sticks" | "caviar"
The syntax for data declarations does not allow such, you must have constructors applied to types. You could make it data Wine = Red Red | White White data Red = Merlot | Syrah data White = SauvignonBlanc | Riesling | PinotNoir data Entree = Pork | Chicken | Tuna deriving (Eq, Enum, Bounded) data SideDish = GarlicBread | MozzarellaSticks | Caviar deriving (Eq, Enum, Bounded) instance Show Wine where ... instance Show Red where ... ...
Then, I want to write a Haskell function that takes two lists; e.g.,
["pork", "chicken", "tuna"]
and
["garlic bread", "mozzarella sticks", "caviar"]
and that constructs a three-tuple of the following type:
(Entree, SideDish, Wine)
such that which Wine is chosen depends on the particular combination of Entree and SideDish, with respect to the following rules:
("pork", "garlic bread") -> ("pork", "garlic bread", "Merlot") ("pork", "mozzarella sticks") -> ("pork", "mozzarella sticks", "Merlot") ("pork", "caviar") -> ("pork", "Beluga", "Sauvignon Blanc") ("chicken", "garlic bread") -> ("chicken", "garlic bread", "Merlot") ("chicken", "mozzarella sticks") -> ("chicken", "mozzarella sticks", "Sauvignon Blanc") ("chicken", "caviar") -> ("chicken", "caviar", "Merlot") ("tuna", "garlic bread") -> ("tuna", "garlic bread", "Sauvignon Blanc") ("tuna", "mozzarella sticks") -> ("tuna", "mozzarella sticks", "Merlot") ("tuna", "caviar") -> ("tuna", "caviar", "Merlot")
selectWine :: Entree -> SideDish -> Wine selectWine Pork sd = case sd of Caviar -> White SauvignonBlanc _ -> Red Merlot selectWine Chicken sd = case sd of MozzarellaSticks -> White SauvignonBlanc _ -> Red Merlot selectWine Tuna sd = case sd of GarlicBread -> White SauvignonBlanc _ -> Red Merlot options :: [(Entree,SideDish,Wine)] options = [(e,s,selectWine e s) | e <- [minBound .. maxBound], s <- [minBound .. maxBound]]
So far, I have written the following Haskell code:
module Wine where
data Wine = Red | White data Red = "Merlot" data White = "Sauvignon Blanc" data Entree = "pork" | "chicken" | "tuna" data SideDish = "garlic bread" | "mozzarella sticks" | "caviar"
wine :: [Entree] -> [SideDish] -> [(Entree, SideDish, Wine)] wine entree sidedish
| entree == "pork" = | | sidedish == "garlic bread" = ("pork", "garlic bread",
"Merlot")
| sidedish == "mozzarella sticks" = ("pork", "mozzarella
sticks", "Merlot")
| sidedish == "caviar" = ("pork", "caviar", "Sauvignon Blanc") | | entree == "chicken" = | | sidedish == "garlic bread" = ("chicken", "garlic bread",
"Merlot")
| sidedish == "mozzarella sticks" = ("chicken", "mozzarella
sticks", "Sauvignon Blanc")
| sidedish == "caviar"= ("chicken", "caviar", "Merlot") | | entree == "tuna" = | | sidedish == "garlic bread" = ("tuna", "garlic bread",
"Sauvignon Blanc")
| sidedish == "mozzarella sticks" = ("tuna", "mozzarella
sticks", "Merlot")
| sidedish == "caviar"= ("tuna", "caviar", "Merlot")
However, when I load this code into GHCi, I get the following error message:
[1 of 1] Compiling Wine ( wine.hs, interpreted )
wine.hs:18:11: parse error on input `"' Failed, modules loaded: none. Prelude>
I discovered the following seemingly relevant HaskellWiki page, but am not sure about how to apply the advice in it to this problem, because the advice does not seem sufficiently specific or detailed:
List instance - HaskellWiki http://www.haskell.org/haskellwiki/List_instance
This isn't relevant here, you're not trying to create an instance of some typeclass for a list-type.
What would be a good way to overcome this error?
-- Benjamin L. Russell
See above, but I suggest rethinking your menu :)