[Haskell-beginners] showing user defined data types

[Dan Lior]

Hello, 

I defined a type Pixel :

data Pixel = Black | White deriving (Eq, Show)

and would like to "show" it as 

show :: Pixel -> String
show Black = "#"
show White = "."

Apparently, ghc finds this ambiguous. That seems strange to me since Pixel is my own custom type and I don't imagine that ghc has another definition for showing it. Isn't this what "overloading" is supposed to enable?

I can get around the problem by :

import Prelude hiding(show)

but that seems to drastic since it hides all the various overloaded incarnations of "show". I only need it to hide one of them (the one for Pixel). 

I'm obviously misunderstanding something. 

Is there a simple way to define my own show function for my own user defined data type? 

Thanks in advance. 

dan