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objects: the lack of method overloading (looking for a way around it)
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Date: -- (:)
From: philippe <kelvin.bitnick@g...>
Subject: Re: objects: the lack of method overloading (looking for a way around it)
philippe,

use parametrized classes.

--
philippe

On Mon, Oct 4, 2010 at 7:32 PM, philippe <kelvin.bitnick@gmail.com> wrote:
> Hello,
>
> I'm just beginning a real, usefull usage of the object system in ocaml.
> I'm a bit stalled on one thing, it's the lack of method overloading like in C++.
> My need is in a case of building a rather simple pipeline pattern:
> each instance of an element (a node) in the pipeline can get a
> numerical value from the previous node, do some computation on it, and
> store it locally in a hash for caching/efficiency, as returning it
> using the #get method.
>
> The real need see most of the time a 1D array of float (a vector) in
> input and output of each node.
> but sometime a Complex.t array.
>
> So I need some kind of polymorphism for allowing :
>
> float array -> float array
> Complet.t array -> float array
> float array -> Complex.t array
> Complex.t array -> Complex.t array
>
> Here's a shorter (a bit dumb) demo code using int -> int.
>
> (I would much prefer avoid to store float in the real part of a
> complex array - in the real (genuine) code the array may be long,
> 100'000 elements and more, it's code a bit sensitive about
> performances).
>
> Look to the comment MY QUESTION in the definition of the process
> method in the class node_plus
>
> THANK YOU !
>
> --
> Philippe Strauss
>
> --- 8< ---
>
> open Common
> open Log
>
> (* the real code has more usefull type here *)
> type node_parameters = Plus1 | Plus2
>
> (* a class for storing parameters usefull for the whole pipeline chain *)
> class parameters =
>    object(self)
>        (* useless here - usefull in the real code *)
>        val mutable sf = 44100
>        (* a value we will add in a demo node *)
>        val mutable add = 1
>        method get_sf = sf
>        method set_sf sfreq =
>            sf <- sfreq ;
>        method get_add = add
>        method set_add ntoadd =
>            add <- ntoadd
>        (* this is for the cache part of each node: we compute a hash
> of parameter to distinguish hash entries btwn parameters change *)
>        method get_parh np (node_id: int) =
>            let md5 s = Digest.to_hex (Digest.string s) in
>            match np with
>            (* the real code doesn't use always (sf, add) but
> parameter influing the processing of a node *)
>            | Plus1 -> md5 (Marshal.to_string (sf, add) [Marshal.No_sharing])
>            | Plus2 -> md5 (Marshal.to_string (sf, add) [Marshal.No_sharing])
> end
>
> (* main code of a node: a boileplate around a hash to cache computed value *)
> class virtual node_virt prev parm hn =
>    object(self)
>        (* instance of a previous node in the pipeline *)
>        val previous = prev
>        (* instance of the parameters *)
>        val parms = parm
>        (* the hash for caching *)
>        val hres = Hashtbl.create hn
>        (* a serial, bumped up plus one on each cache miss *)
>        val mutable id = 0
>        method get_id = id
>        method inc_id =
>            id <- id + 1
>        (* for passing to (parms #get_parh (self #ptyp ())) in the
> real code, when adding a result into the hash *)
>        method virtual ptyp: unit -> node_parameters
>        (* the real computation is defined in a non virtual method
> later, as simply as possible *)
>        method virtual process: int -> int
>        (* process and store: call process, then store in cache.
> called on a cache miss *)
>        method private pstore: (int * int) =
>            let prid, data = prev #get in
>            (* processing *)
>            let new_data = self #process data in
>            (* we're in a cache miss: bump the serial +1 *)
>            self #inc_id ;
>            let myid = self #get_id in
>            Hashtbl.add hres (parms #get_parh Plus1 myid) new_data ;
>            (myid, new_data)
>        method get =
>            try
>                (* memoize id to avoid race cond *)
>                let myid = self #get_id in
>                (* here again in the real code, we lookup the hash
> using (parms #get_parh (self #ptyp ()) myid) *)
>                let data = Hashtbl.find hres (parms #get_parh Plus1 myid) in
>                (myid, data)
>            with Not_found -> Printf.printf "node_virt #get: cache
> miss!\n%!"; self #pstore
> end
>
> (* a trivial node: add parm #get_add to the integer we receive from
> the previous node *)
> class node_plus prev parm hn =
>    object(self)
>        inherit node_virt prev parm hn
>        method ptyp () = Plus1
>        (*
>            MY QUESTION: what is the best way to allow not only int ->
> int for this process method,
>            but also: int -> float, float -> int, float -> float ?
>            The method minimizing the code written in this non virtual
> node definition, since I may end with 15 or 30
>            differents derived class of node_virt like this single one ?
>        *)
>        method process n =
>            n + (parm #get_add)
> end
>
> (* a front of line, head node having a get method of the same type
> than those derived from node_virt *)
> class node_head parm =
>    object(self)
>        val parms = parm
>        method get =
>            (0, 1)
> end
>
>
> let () =
>    let p = new parameters in
>    p #set_sf 44100 ; (* useless indeed, just for demo *)
>    p #set_add 2 ; (* usefull *)
>    let n0 = new node_head p in
>    let n1 = new node_plus n0 p 3 in
>    Printf.printf "%d\n" (snd (n0 #get)) ;
>    Printf.printf "%d\n" (snd (n1 #get)) ;
>    Printf.printf "%d\n" (snd (n1 #get)) ;
>    p #set_add 1 ;
>    Printf.printf "%d\n" (snd (n1 #get))
>