Haskell中一个简单的面向对象的类'Point'

嗯,我确实认识到我对haskell感到困惑,这是我第一个周末.

我只是想知道以下设计的OO级Point2D

应该用Haskell编写如下:

import Prelude hiding ( sum )

-- ...............................................................
-- "class type"  : types belonging to this family of types
--                              must implement distance and sum functions
-- ...............................................................
class PointFamily p where
    -- p is a type of this family, not a point
    distance :: p -> p -> Float -- takes two things of type p and returns a Real
    sum :: p -> p -> p -- takes two things of type p and returns a p thing

-- ...............................................................
-- data type:  Point2D
--              a new type with x and y coordinates
-- ...............................................................
data Point2D = Point2D { x :: Float , y :: Float }
    deriving (Show) -- it is "showable/printable"

-- ...............................................................
-- Point2D belongs to PointFamily, so let's say it and
-- how to compute distance and sum for this type
-- ...............................................................
instance PointFamily Point2D where

    -- ............................................................-
    distance p1 p2 = sqrt (dx*dx + dy*dy)
        where 
            dx = (x p1) - (x p2)
            dy = (y p1) - (y p2)

    -- ............................................................-
    sum p1 p2 = Point2D { x = (x p1)+(x p2), y = (y p1)+(y p2) }

-- ...............................................................
-- global constant
-- ...............................................................
origin  =  Point2D 0.0 0.0

-- ...............................................................
-- main
-- ...............................................................
main =  do
    putStrLn "Hello"
    print b
    print $ distance origin b
    print $ sum b b 

    where
            b = Point2D 3.0 4.0

是的,我知道我不应该尝试在Haskell中"思考OOP",但是......好吧,1)这需要很长时间,2)在实践中我猜你会发现几个OOP设计需要重写在哈斯克尔