Re: [Haskell-beginners] the equivalent of OO class data and methods

2 Apr 2009

      Thanks. One question:

Yitzchak Gale wrote:
...
One way, which is most similar to the OO approach, is to use
encapsulation.

...
The other approach is using the class system:
class Coordinates a where
  x :: a -> Int
  y :: a -> Int
data LayoutItem = LayoutItem { layoutItemX, layoutItemY :: Int }
instance Coordinates LayoutItem where
  x = layoutItemX
  y = layoutItemY
data Notehead = Notehead { noteheadItemX, noteheadItemY :: Int }
instance Coordinates Notehead where
  x = NoteheadX
  y = NoteheadY
In this case, do local variables still shadow or hide the functions 
contained by the class? In other words, would this work?

foo :: LayoutItem -> Notehead -> Int
foo l n = let y = x l + x n
               z = y l + y n
           in z + y

I'm guessing that the variable y will shadow the function y. This is a 
problem, because I want generic names available for variables.

Now, using encapsulation in modules, I guess it would look like this:

import qualified LayoutItem (x, y)
import LayoutItem ( LayoutItem )
import qualified Notehead (x, y)
import Notehead ( Notehead )

foo :: LayoutItem -> Notehead -> Int

foo l n = let y = LayoutItem.x l + Notehead.x n
               z = LayoutItem.y l + Notehead.y n
           in z + y

Re: [Haskell-beginners] the equivalent of OO class data and methods

Michael P Mossey