1 The problem
Here's a simple test for object orientation (for some reasonable definition):
- Define a type A such that for any type B you can define
up :: B -> A down :: A -> Maybe B
- such that
down . up = Just
You can do this quite easily in Java or C++, mutatis mutandis. You can't do this in Haskell, I don't think. You can't actually do this in O'Haskell either, it seems the O' essentially amounts to syntactic sugar.
You can do a weaker form of this with Haskell's Dynamic, where you only have to deal with Bs that are instances of Typeable. But even with that, note that Dynamic/Typeable/TypeRep are a bit messy, with instances for Typeable defined for a wide range of known types.
An alternative approach would be to identify your B within A not per-B but per-(up,down). This would allow for instance separate (up,down) for the same B such that
down1 . up2 = Nothing down2 . up1 = Nothing
Of course this can be done with Dynamic too, by defining dummy types. But it's ugly.
2 Extensible datatypes
Extensible datatypes allow a type to be defined as "open", which can later be extended by disjoint union. Here's a sample syntax that achieves the above OO test:
module P where data A = ..
module Q where import P
A |= MkB B
up = MkB down (MkB b) = Just b down _ = Nothing
3 Deriving Dynamic
It's possible to define Dynamic using extensible datatypes. Here's a naive attempt:
data Dynamic = ..
class Typeable' a where toDyn :: a -> Dynamic fromDynamic :: Dynamic -> Maybe a
-- for each type...
Dynamic |= MkBool Bool
instance Typeable' Bool where toDyn = MkBool fromDynamic (MkBool b) = Just b fromDynamic _ = Nothing
This attempt however doesn't allow easy creation of Typeable1, Typeable2 etc. A better way is to use type-constructor parameters:
data Dynamic0 (f :: * -> *) = ..
data Dynamic1 (g :: (* -> *) -> *) = ..
type Dynamic = Dynamic0 Identity
data Type a = MkType
type TypeRep = Dynamic0 Type
class Typeable0 a where toDyn0 :: f a -> Dynamic0 f fromDynamic0 :: Dynamic0 f -> Maybe (f a)
class Typeable1 p where toDyn1 :: g p -> Dynamic1 g fromDynamic1 :: Dynamic1 g -> Maybe (g p)
data Compose p q a = MkCompose (p (q a)) data Compose1 d0 f p = MkCompose1 (d0 (Compose f p))
Dynamic0 f |= MkDynamic1 (Dynamic1 (Compose1 Dynamic0 f))
unDynamic1 :: Dynamic0 f -> Maybe (Dynamic1 (Compose1 Dynamic0 f)) unDynamic1 (MkDynamic1 xx) = Just xx unDynamic1 _ = Nothing
instance (Typeable1 p,Typeable0 a) => Typeable0 (p a) -- toDyn0 :: f (p a) -> Dynamic0 f toDyn0 = MkDynamic1 . toDyn1 . MkCompose1 . toDyn0 . MkCompose -- fromDynamic0 :: Dynamic0 f -> Maybe (f (p a)) fromDynamic0 dyn = do dcdf <- unDynamic1 dyn (MkCompose1 dcfp) <- fromDynamic1 dcdf (MkCompose fpa) <- fromDynamic0 dcfp return fpa
-- for each type
Dynamic0 f |= MkInt (f Int)
instance Typeable0 Int where toDyn0 = MkInt fromDynamic0 (MkInt fi) = Just fi fromDynamic0 _ = Nothing
Dynamic1 g |= MkMaybe (g Maybe)
instance Typeable1 Maybe where toDyn1 = MkMaybe fromDynamic1 (MkMaybe gm) = Just gm fromDynamic1 _ = Nothing
I submit that this is "hairy" rather than "ugly", but I suspect the Type-Constructors Of Unusual Kind (TCOUKs) get even hairier for Typeable2, Typeable3 etc...