If you want a simple translation, use Word8 (from Data.Word) for the type and use Data.Bits for operations on it just like in C++. This would offer you storage efficiency (if stored as a strict field).

If you want idiomatic Haskell, constructor of the form:

  data ObjectType = Object | Item | Container | Unit | Player, etc.

Then simply put intelligence into the `isContainer` or `toWord8`, etc. translations. This latter approach will be more extensible in the long run, since you might find you want some parameterized object types.

Re: OO Classes and Haskell 

If your classes are more like `interfaces`, you could use Typeclasses to model them. Otherwise, look into OOHaskell. But I think your program architecture will simply be different in idiomatic Haskell than in idiomatic C++. 

Regards,

Dave


On Mon, Jan 23, 2012 at 1:14 PM, Daniel Hlynskyi <abcz2.uprola@gmail.com> wrote:
Thanks. This and previous email are answers to question I asked. But not the answer to question I mean.
I'll describe the whole task, as Yves Parès suggested.

I'm trying to convert C++ code to Haskell. I have such hierarchy: class Object, class Item : Object, class Container : Item. Another one example: class Unit : Object, class Player : Unit. Each constructor do things like this:

Object::Object()



{



    objectType        = TYPEMASK_OBJECT;



    // ... lots of code ...


}

Item::Item()



{



    objectType |= TYPEMASK_ITEM;



    // ...


}



Container::Container(): Item()



{



    objectType |= (TYPEMASK_ITEM | TYPEMASK_CONTAINER);



    // ...


}

What is objectType? This field is used when a networksend packet is created. In the packet it is 1 byte of flags, so it is in object hierarchy.

So the question was: what type should objectType field have in Haskell? I think it must not mimic enum. What the structure have I to use? There is one more problem - there may be lots of objects, lots of, so memory efficiency is also suggested.
And side question: what to do with classes? =) Maybe there is simple rule to convert OO hierarchy to FP.

23 січня 2012 р. 12:15 Malcolm Wallace <malcolm.wallace@me.com> написав:


> 2012/1/22 Данило Глинський <abcz2.uprola@gmail.com>
> What is natural Haskell representation of such enum?
>
> enum TypeMask
> {
>    UNIT,
>    GAMEOBJECT,
>
>    CREATURE_OR_GAMEOBJECT = UNIT | GAMEOBJECT
> };

I don't think that definition makes any sense in C, because UNIT is 0, so UNIT | GAMEOBJECT == GAMEOBJECT == 1

Nevertheless, in Haskell something vaguely similar might be:

data TypeMask = UNIT | GAMEOBJECT | CREATURE_OR_GAMEOBJECT

> // 1-byte flaged enum
> enum TypeMask
> {
>    // ...
>    UNIT        = 0x0004,
>    GAMEOBJECT  = 0x0008,
>    // ...
>
>    CREATURE_OR_GAMEOBJECT = UNIT | GAMEOBJECT
>    WORLDOBJECT = UNIT | PLAYER | GAMEOBJECT | DYNAMICOBJECT | CORPSE
>    // ... even more enum combos ...
> };

import Data.Bits
data TypeMask = UNIT | GAMEOBJECT | CREATURE_OR_GAMEOBJECT | WORLDOBJECT
instance Enum TypeMask where
   fromEnum UNIT = 0x4
   fromEnum GAMEOBJECT = 0x8
   fromEnum CREATURE_OR_GAMEOBJECT = fromEnum UNIT .|. fromEnum GAMEOBJECT
   fromEnum WORLDOBJECT = fromEnum UNIT .|. fromEnum PLAYER .|. fromEnum GAMEOBJECT
                          .|. fromEnum DYNAMICOBJECT .|. fromEnum CORPSE

   toEnum 0x4 = UNIT
   toEnum 0x8 = GAMEOBJECT
   toEnum _   = error "unspecified enumeration value of type TypeMask"

isCreatureOrGameObject :: Int -> Bool
isCreatureOrGameObject x = (x .|. fromEnum CREATURE_OR_GAMEOBJECT) /= 0

isWorldObject :: Int -> Bool
isWorldObject x = (x .|. fromEnum WORLDOBJECT) /= 0

-- But fundamentally, this is not an idiomatic Haskell way of doing things.
-- The other posts in this thread have shown more Haskell-ish translations.




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