[Haskell-beginners] Re: [Haskell-cafe] represent data sturcture
using function
Ryan Ingram
ryani.spam at gmail.com
Mon Dec 29 04:50:02 EST 2008
Bonus points if you find the stupid bug in my code :)
-- ryan
On Mon, Dec 29, 2008 at 1:48 AM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> On Sun, Dec 28, 2008 at 10:13 PM, David Menendez <dave at zednenem.com> wrote:
>> 2008/12/29 Raeck chiu <raeck at msn.com>:
>>> It seems to be very difficult to change the number of Male or Female if a
>>> concrete data structure is not used. Is it possible change the number of Male in classA
>>> when represent classA using function?
>>
>> Here's the simplest way:
>>
>> update k v map = \k' -> if k' == k then v else map k'
>>
>> Note that it requires an Eq instance for Sex.
>>
>> let classA' = update Male 150 classA
>> in (classA' Male, classA' Female)
>> =
>> (150,200)
>
> Of course this version of update leaks crazy amounts of memory:
>
>> let bigmap = iterate (update Male 150) classA !! 100000
>> bigmap Male
>
> "bigmap" leaves a huge chain of thunks pointing at each other, which
> can never be freed.
>
> Using functions is mathematically more elegant than some concrete data
> structure (which might require Eq or Ord or other constraints, and
> have multiple observable representations for the same map, or have
> maps that don't include every element).
>
> However, "generic maps" have been improving a lot recently, especially
> with data families in the new GHC. You use an abstract type (k :->
> v) to represent the map; this type is semantically equivalent to (k ->
> v) via some observation function for generic maps, but has a compact
> representation. For example:
>
>> class MapKey k where
>> data k :-> v
>> newMap :: (k -> v) -> (k :-> v)
>> fetch :: (k :-> v) -> (k -> v)
>> update :: (k,v) -> (k :-> v) -> (k :-> v)
>> empty :: v -> (k :-> v)
>> empty = newMap (const v)
>
>> instance MapKey Bool where
>> data Bool :-> v = BoolMap v v
>> newMap f = BoolMap (f False) (f True)
>> fetch (Boolmap t f) v = if v then t else f
>> update (True, t) (BoolMap _ f) = Boolmap t f
>> update (False, f) (BoolMap t _) = Boolmap t f
>
> "fetch" converts this representation of a total function over k, into
> an actual function. The representation of k :-> v might vary
> depending on k; Int might use IntMap, (k1,k2) can compose maps:
>
>> instance (MapKey k1, MapKey k2) => MapKey (k1,k2) where
>> newtype (k1,k2) :-> v = PairMap (k1 :-> (k2 :-> v))
>> ...
>
>> instance (MapKey k1, MapKey k2) => MapKey (Either k1 k2) where
>> data (Either k1 k2) :-> v = EitherMap (k1 :-> v) (k2 :-> v)
>> ...
>
> This gives you the same benefits as the function approach but without
> the terrible performance issues. You do need to write instances for
> your types, though, although most can be easily derived from the
> instances for pairs, Either, and Integer.
>
> See http://www.haskell.org/haskellwiki/GHC/Indexed_types for more.
>
> -- ryan
>
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