Am Donnerstag, 23. Oktober 2008 12:19 schrieb Benjamin L.Russell:
You have basically two choices, you can say everybody knows which colour the wines have and use
instance Show Wine where show (Red w) = show w show (White w) = show w
or include the colour in the rendered String, for example via deriving Show, or by declaring your own instance (if you want White PinotNoir rendered as "White (Pinot Noir)", you would have to do that or write a more complicated Show instance for White). If you want to parse values of type Wine in the future, it will be slightly easier with the dataconstructors included.
So perhaps I should rewrite the type definitions for Red and White as follows?
data Red = Syrah | Merlot | Port deriving (Eq, Show) data White = SauvignonBlanc | Riesling | PinotNoir deriving (Eq, Show)
That would be the easiest, but of course having the two Whites displayed with a space is aesthetically superior.
Alternatively (adapted from an example provided by Dirk Thierbach in the thread "seeking helpful links," dated "Tue, 21 Oct 2008 16:33:41 -0700 (PDT)," on comp.lang.haskell), perhaps the following?
type Entree = String type SideDish = String type Wine = String
wine :: Entree -> SideDish -> Wine wine "Kobe Beef" "Buttered Potatoes" = "Syrah" wine "Kobe Beef" "Brie Cheese" = "Syrah" wine "Kobe Beef" "Greek Salad" = "Sauvignon Blanc" wine "Lemon Chicken" "Buttered Potatoes" = "Merlot" wine "Lemon Chicken" "Brie Cheese" = "Sauvignon Blanc" wine "Lemon Chicken" "Greek Salad" = "Merlot" wine "Steamed Salmon" "Buttered Potatoes" = "Sauvignon Blanc" wine "Steamed Salmon" "Brie Cheese" = "Port" wine "Steamed Salmon" "Greek Salad" = "Port"
wines :: [Entree] -> [SideDish] -> [(Entree, SideDish, Wine)] wines entrees sideDishes = [(e, s, wine e s) | e <- entrees, s <- sideDishes]
The downside of that is that a typo in the entrees or sideDishes will explode with a pattern match failure. Otherwise it's concise and clean.
[...]
Bon Appetit, Daniel
P.S.: Could I have a Syrah with my beef and potatoes?
Certainly; here is your Syrah with your beef and potatoes. Bon appetit ;-) :
Merci, c'était délicieux.
-- Benjamin L. Russell