Clojure Protocols & expression problem

[retard]

Wed, 28 Apr 2010 13:22:53 -0400, bearophile wrote:

> BCS:
> 
>>Could you elaborate on what exactly the expression problem is?<
> 
> I am far from being an expert on such matters, I will do what I can to
> answer. It's not an esoteric problem, if you program with an OO language
> you have probably faced it sometime.

You failed to mention that the OOP solution is in many cases overly 
complex. When you don't need to subclass the car elements, a functional 
pattern matching happens to win - at least if the declarativeness of code 
matters. The implementation can also use jump tables so it becomes faster 
than double dispatch. Luckily D is never going to get algebraic data 
types so you can all stop reading this post right here.

data CarElement = Wheel Name
                | Engine
                | Body
                | Car [CarElement]

defaultCar = Car [ Wheel "front left"
                 , Wheel "front right"
                 , Wheel "back left"
                 , Wheel "back right"
                 , Body
                 , Engine
                 ]

makeVisitor f element = map f (case element of
   Car elements = element : elements
   _            = [element]
)


visitor element = case element of
   Wheel name = "Visiting " +++ name +++ " wheel"
   Engine     = "Visiting engine"
   Body       = "Visiting body"
   Car _      = "Visiting car"

doVisitor element = case element of
   Wheel name = "Kicking my " +++ name +++ " wheel"
   Engine     = "Starting my engine"
   Body       = "Moving my body"
   Car _      = "Starting my car"

-- an example of usage:

-- makeVisitor visitor default
-- makeVisitor doVisitor default