Difference between revisions of "Extensible datatypes"
m (→The problem) |
m (fmt) |
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:Define a type <tt>A</tt> such that for any type <tt>B</tt> you can define |
:Define a type <tt>A</tt> such that for any type <tt>B</tt> you can define |
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+ | <haskell> |
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− | up :: B -> A |
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− | + | up :: B -> A |
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+ | down :: A -> Maybe B |
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+ | </haskell> |
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:such that |
:such that |
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+ | <haskell> |
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− | + | down . up = Just |
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+ | </haskell> |
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You can do this quite easily in Java or C++, ''mutatis mutandis''. You can't do this in Haskell (or [[O'Haskell]] either). |
You can do this quite easily in Java or C++, ''mutatis mutandis''. You can't do this in Haskell (or [[O'Haskell]] either). |
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An alternative approach would be to identify your <tt>B</tt> within <tt>A</tt> not per-<tt>B</tt> but per-(up,down). This would allow for instance separate (up,down) for the same <tt>B</tt> such that |
An alternative approach would be to identify your <tt>B</tt> within <tt>A</tt> not per-<tt>B</tt> but per-(up,down). This would allow for instance separate (up,down) for the same <tt>B</tt> such that |
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+ | <haskell> |
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⚫ | |||
− | + | down1 . up2 = Nothing |
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⚫ | |||
+ | </haskell> |
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Of course this can be done with <tt>Dynamic</tt> too, by defining dummy types. But it's ugly. |
Of course this can be done with <tt>Dynamic</tt> too, by defining dummy types. But it's ugly. |
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'''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: |
'''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: |
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+ | <haskell> |
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− | + | module P where |
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− | + | data A = .. |
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− | + | module Q where |
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− | + | import P |
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− | + | A |= MkB B |
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− | + | up = MkB |
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− | + | down (MkB b) = Just b |
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− | + | down _ = Nothing |
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+ | </haskell> |
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== Deriving Dynamic == |
== Deriving Dynamic == |
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It's possible to define [[Dynamic]] using extensible datatypes. Here's a naive attempt: |
It's possible to define [[Dynamic]] using extensible datatypes. Here's a naive attempt: |
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+ | <haskell> |
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− | + | data Dynamic = .. |
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− | + | class Typeable' a where |
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− | + | toDyn :: a -> Dynamic |
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− | + | fromDynamic :: Dynamic -> Maybe a |
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− | + | -- for each type... |
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− | + | Dynamic |= MkBool Bool |
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+ | |||
⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
+ | </haskell> |
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⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
This attempt however doesn't allow easy creation of <tt>Typeable1</tt>, <tt>Typeable2</tt> etc. A better way is to use type-constructor parameters: |
This attempt however doesn't allow easy creation of <tt>Typeable1</tt>, <tt>Typeable2</tt> etc. A better way is to use type-constructor parameters: |
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+ | <haskell> |
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− | + | data Dynamic0 (f :: * -> *) = .. |
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− | + | data Dynamic1 (g :: (* -> *) -> *) = .. |
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− | + | type Dynamic = Dynamic0 Identity |
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− | + | data Type a = MkType |
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− | + | type TypeRep = Dynamic0 Type |
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− | + | class Typeable0 a where |
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− | + | toDyn0 :: f a -> Dynamic0 f |
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− | + | fromDynamic0 :: Dynamic0 f -> Maybe (f a) |
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− | + | class Typeable1 p where |
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− | + | toDyn1 :: g p -> Dynamic1 g |
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− | + | fromDynamic1 :: Dynamic1 g -> Maybe (g p) |
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− | + | data Compose p q a = MkCompose (p (q a)) |
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− | + | data Compose1 d0 f p = MkCompose1 (d0 (Compose f p)) |
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− | + | Dynamic0 f |= MkDynamic1 (Dynamic1 (Compose1 Dynamic0 f)) |
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− | + | unDynamic1 :: Dynamic0 f -> Maybe (Dynamic1 (Compose1 Dynamic0 f)) |
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− | + | unDynamic1 (MkDynamic1 xx) = Just xx |
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− | + | unDynamic1 _ = Nothing |
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− | + | instance (Typeable1 p,Typeable0 a) => Typeable0 (p a) |
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− | + | -- toDyn0 :: f (p a) -> Dynamic0 f |
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− | + | toDyn0 = MkDynamic1 . toDyn1 . MkCompose1 . toDyn0 . MkCompose |
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− | + | -- fromDynamic0 :: Dynamic0 f -> Maybe (f (p a)) |
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− | + | fromDynamic0 dyn = do |
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− | + | dcdf <- unDynamic1 dyn |
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− | + | (MkCompose1 dcfp) <- fromDynamic1 dcdf |
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− | + | (MkCompose fpa) <- fromDynamic0 dcfp |
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− | + | return fpa |
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− | + | -- for each type |
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− | + | Dynamic0 f |= MkInt (f Int) |
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− | + | instance Typeable0 Int where |
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− | + | toDyn0 = MkInt |
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− | + | fromDynamic0 (MkInt fi) = Just fi |
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− | + | fromDynamic0 _ = Nothing |
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− | + | Dynamic1 g |= MkMaybe (g Maybe) |
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− | + | instance Typeable1 Maybe where |
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− | + | toDyn1 = MkMaybe |
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− | + | fromDynamic1 (MkMaybe gm) = Just gm |
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− | + | fromDynamic1 _ = Nothing |
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+ | </haskell> |
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I submit that this is "hairy" rather than "ugly", but I suspect the Type-Constructors Of Unusual Kind (TCOUKs) get even hairier for <tt>Typeable2</tt>, <tt>Typeable3</tt> etc... |
I submit that this is "hairy" rather than "ugly", but I suspect the Type-Constructors Of Unusual Kind (TCOUKs) get even hairier for <tt>Typeable2</tt>, <tt>Typeable3</tt> etc... |
Revision as of 23:05, 2 April 2006
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 (or O'Haskell either).
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.
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
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...