Fw: [Caml-list] Missing a function

Fw: [Caml-list] Missing a function

[Frédéric Gava]

Hi,
>
> > Another option is to build on top of the stdlib Map functor another
> > functor that keeps track of the cardinal of a map :
> >
> > module CMap = functor (Ord : Map.OrderedType) ->
> >   (struct
> >     module M = Map.Make (Ord)
> >
> >     type key = M.key
> >     type 'a t = int * 'a M.t
> >
> >     let cardinal = fst
> >
> >     let empty = 0, M.empty
> >
> >     let add k v (n, m) = (n + 1, M.add k v m)
> >
> >     let find k (_, m) = M.find k m
> >
> > ....
> >    : sig
> >        include Map.S
> >        val cardinal : 'a t -> int
> >      end)
> >
Hum. To remove an element, you have to test is its exists or not before
decrease n. It could be to much cost expansive...no ?

Regards,
Frédéric Gava

Re: [Caml-list] Missing a function

[Olivier Andrieu]

 > Frédéric Gava [Sun, 30 Jan 2005]:
 > Hi,
 > >
 > > > Another option is to build on top of the stdlib Map functor another
 > > > functor that keeps track of the cardinal of a map :
 > > >
 > > > module CMap = functor (Ord : Map.OrderedType) ->
 > > >   (struct
 > > >     module M = Map.Make (Ord)
 > > >
 > > >     type key = M.key
 > > >     type 'a t = int * 'a M.t
 > > >
 > > >     let cardinal = fst
 > > >
 > > >     let empty = 0, M.empty
 > > >
 > > >     let add k v (n, m) = (n + 1, M.add k v m)
 > > >
 > > >     let find k (_, m) = M.find k m
 > > >
 > > > ....
 > > >    : sig
 > > >        include Map.S
 > > >        val cardinal : 'a t -> int
 > > >      end)
 > > >
 > Hum. To remove an element, you have to test is its exists or not
 > before decrease n. It could be to much cost expansive...no ?

Oh right. And a test is needed for `add' too since the cardinal will
not change if the key is already bound in the map. So, all in all it's
not a very good idea :)

-- 
   Olivier