# Extensible datatypes

### From HaskellWiki

## 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...