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

23 Oct 2008

      On Thu, 23 Oct 2008 18:39:52 +0900, Benjamin L. Russell
 wrote:
...
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"
The only remaining issue is whether there is a way to define the type
constructor for Wine without pre-defining parts that are later defined
in the type constructors for Red and White.

In the above code, for example,
...
instance Show Wine where 
   show (Red Merlot) = "Merlot"
essentially pre-defines the following definition later given in the
type constructor for Red:
...
instance Show Red where
   show Merlot = "Merlot"
This seems redundant.

Since the type definition for Wine is 

data Wine = Red Red | White White

the type constructors for Red and White both require an argument, so 

show (Red Merlot) = ...

seems reasonable.

This would seem to imply that if I need to reduce redundancy, I should
probably rewrite the RHS of the above line.

Is there a way to refer to the type constructors for Red and White in
the type constructor for Wine?

-- Benjamin L. Russell

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

Benjamin L.Russell