Re: [Haskell-cafe] [C][enums][newbie] What is natural Haskell representation of such enum?

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 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 написав:

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2012/1/22 Данило Глинський 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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