Re: [Caml-list] tip for tail recursive map

[Christophe Raffalli <christophe.raffalli@univ-savoie.fr>]

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> 
> You can do better with an ad-hoc encoding of the continuation
> instead of using closures:
> 
> let rec map k f = function
>   | [] -> List.rev k
>   | x :: rl -> map (f x :: k) f rl
> ;;
> 
> The memory footprint is smaller, and you spend much less time
> invoking closures.
> 
> Note that I haven't bothered benchmarking these two functions.

I did the benchmark with four version of map (below):

List of size 10, 10000000 times with standard map : 0.948059s
List of size 10, 10000000 times with map with rev : 1.800112s
List of size 10, 10000000 times with map with prelist : 3.060192s
List of size 10, 10000000 times with map with obj : 1.704105s

List of size 100, 1000000 times with standard map : 1.068068s
List of size 100, 1000000 times with map with rev : 1.448090s
List of size 100, 1000000 times with map with prelist : 2.668166s
List of size 100, 1000000 times with map with obj : 1.652104s

List of size 1000, 100000 times with standard map : 1.792112s
List of size 1000, 100000 times with map with rev : 2.912182s
List of size 1000, 100000 times with map with prelist : 3.520220s
List of size 1000, 100000 times with map with obj : 2.460154s

List of size 10000, 10000 times with standard map : 7.564473s
List of size 10000, 10000 times with map with rev : 15.452965s
List of size 10000, 10000 times with map with prelist : 12.672792s
List of size 10000, 10000 times with map with obj : 11.572724s

List of size 100000, 1000 times with standard map : 33.018063s
List of size 100000, 1000 times with map with rev : 42.142634s
List of size 100000, 1000 times with map with prelist : 22.161385s
List of size 100000, 1000 times with map with obj : 20.801299s

standard map with size 1000000 segfaults on my machine
List of size 1000000, 100 times with map with rev : 55.211450s
List of size 1000000, 100 times with map with prelist : 23.549472s
List of size 1000000, 100 times with map with obj : 21.777361s

standard map = List.map

map with rev = the above code given by Damien Doligez

map with prelist = a dirty map using Obj but through a
  not too dirty, but not completely safe interface prelist (attached) :

let pmap f l =
  let pl = start () in
  let rec fn = function
    [] -> Prelist.extract pl
  | a::l -> Prelist.cons (f a) pl; fn l
  in
  fn l

map with obj : a directly dirty obj map :

let objmap f l =
  match l with
    [] -> []
  | x::l' ->
      let start = [f x] in
      let rec fn current = function
	  [] -> start
	| x::l' ->
	    let l'' = [f x] in
	    Obj.set_field (Obj.repr current) 1 (Obj.repr l'');
	    fn l'' l'
      in
      fn start l'

Conclusion : dirty map wins for large lists, Standard map wins for small lists,
but if you map a non trivial function (here I map the trivial succ function on int),
there should be no difference so I would suggest to use map with reverse.

The conclusion is that I would replace Xavier's suggestion in another thread :
 << Repeat after me: "Obj.magic is not part of the OCaml language". >>
By
 << Try very very very hard not to use object. If it fails try very hard to use Obj but no C. >>

I still think Obj is safer than C (I do not speak for interfacing with external library where C is
mandatory), but you should be aware that Obj needs as much knowledge about the runtime than
C interface without using the provided C macro ...

Cheers,
Christophe

-- 
Christophe Raffalli
Universite de Savoie
Batiment Le Chablais, bureau 21
73376 Le Bourget-du-Lac Cedex

tel: (33) 4 79 75 81 03
fax: (33) 4 79 75 87 42
mail: Christophe.Raffalli@univ-savoie.fr
www: http://www.lama.univ-savoie.fr/~RAFFALLI
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type 'a prelist = { mutable start : 'a list; mutable current : 'a list}

let start () = { start = []; current = []}

let cons a pl = match pl.current with
  [] -> let l = [a] in pl.current <- l; pl.start <- l
| l -> 
	let l' = [a] in Obj.set_field (Obj.repr l) 1 (Obj.repr l');
        pl.current <- l'

let extract pl = 
  let r = pl.start in 
  pl.current <- []; pl.start <- []; 
  (* to guaranty that we can not mute the list once it has been     
  extracted *)
  r

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type 'a prelist (* mutable type of a list under construction *)

val start : unit -> 'a prelist
val extract : 'a prelist -> 'a list
val cons : 'a -> 'a prelist -> unit

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