[Haskell-beginners] Re: how to define a user datatype consisting of instances of String?

23 Oct 2008

      On Thu, 23 Oct 2008 17:51:59 +0900, Benjamin L.Russell
 wrote:
...
[...]
After taking your advice into account, I have rewritten the wine.hs
program as follows, but am unable to figure out what to write for the
type constructor for Wine.  In particular, I do not understand why the
user type definition for Wine is "Red Red | White White" instead of
just "Red | White."
Should I fill out "instance Show Wine where ..." as something similar
to
instance Show Wine where
   show Red = ...
   show White = ...
and then supply a conditional after "show Red = ..." and "show White =
...," or do I need to supply something else there?
[...]
After researching type classes somewhat, I discovered that in "data
Wine = Red Red | White White," the second terms "Red" and "White" were
arguments to their respective first terms of the same name.

With this information, I then rewrote the type definition and type
constructor for Wine as follows:

data Wine = Red Red | White White

instance Show Wine where 
    show (Red Merlot) = "Merlot"
    show (Red Syrah) = "Syrah"
    show (Red Port) = "Port"
    show (White SauvignonBlanc) = "Sauvignon Blanc"
    show (White Riesling) = "Riesling"
    show (White PinotNoir) = "Pinot Noir"

Here is my revised program, which seems to work fine this time:

module Wine where 

data Wine = Red Red | White White
data Red = Merlot | Syrah | Port
data White = SauvignonBlanc | Riesling | PinotNoir
data Entree = KobeBeef | LemonChicken | SteamedSalmon deriving (Eq,
Enum, Bounded)
data SideDish = ButteredPotatoes | BrieCheese | GreekSalad deriving
(Eq, Enum, Bounded)

instance Show Wine where 
    show (Red Merlot) = "Merlot"
    show (Red Syrah) = "Syrah"
    show (Red Port) = "Port"
    show (White SauvignonBlanc) = "Sauvignon Blanc"
    show (White Riesling) = "Riesling"
    show (White PinotNoir) = "Pinot Noir"

instance Show Red where
    show Merlot = "Merlot"
    show Syrah = "Syrah"
    show Port = "Port"

instance Show White where
    show SauvignonBlanc = "Sauvignon Blanc"
    show Riesling = "Riesling"
    show PinotNoir = "Pinot Noir"

instance Show Entree where 
    show KobeBeef = "Kobe Beef"
    show LemonChicken = "Lemon Chicken"
    show SteamedSalmon = "Steamed Salmon"

instance Show SideDish where
    show ButteredPotatoes = "Buttered Potatoes"
    show BrieCheese = "Brie Cheese"
    show GreekSalad = "Greek Salad"

selectWine :: Entree -> SideDish -> Wine
selectWine KobeBeef sd 
    = case sd of
	GreekSalad -> White SauvignonBlanc
	_ -> Red Merlot
selectWine LemonChicken sd
    = case sd of
	BrieCheese -> White Riesling
	_ -> Red Syrah
selectWine SteamedSalmon sd
    = case sd of
	ButteredPotatoes -> White PinotNoir
	_ -> Red Port

options :: [(Entree,SideDish,Wine)]
options = [(e,s,selectWine e s) | e <- [minBound .. maxBound], s <-
[minBound .. maxBound]]

When I run it in GHCi (with my Haskell program file renamed as
wine2.hs), these are the results:

Prelude> :l wine2.hs
[1 of 1] Compiling Wine             ( wine2.hs, interpreted )
Ok, modules loaded: Wine.
*Wine> options
[(Kobe Beef,Buttered Potatoes,Merlot),(Kobe Beef,Brie
Cheese,Merlot),(Kobe Beef,
Greek Salad,Sauvignon Blanc),(Lemon Chicken,Buttered
Potatoes,Syrah),(Lemon Chic
ken,Brie Cheese,Riesling),(Lemon Chicken,Greek Salad,Syrah),(Steamed
Salmon,Butt
ered Potatoes,Pinot Noir),(Steamed Salmon,Brie Cheese,Port),(Steamed
Salmon,Gree
k Salad,Port)]
*Wine>

These results seem correct.

Thank you for your help!

-- Benjamin L. Russell