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Date: Fri, 18 Jan 2008 20:30:59 +0100
From: "Olivier Andrieu" <oandrieu@nerim.net>
Sender: oandrieu@gmail.com
To: "Christian Sternagel" <Christian.Sternagel@uibk.ac.at>
Subject: Re: [Caml-list] camlp4
Cc: caml-list@yquem.inria.fr
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X-Spam: no; 0.00; andrieu:01 oandrieu:01 camlp:01 camlp:01 flatten:01 appends:01 syntax:01 recursive:01 oandrieu:01 ocaml:01 wrote:01 preprocessor:01 caml-list:01 tail:01 nerim:01 

On Jan 18, 2008 6:08 PM, Christian Sternagel
<Christian.Sternagel@uibk.ac.at> wrote:
> When using `camlp4o -parser Camlp4ListComprehension' as preprocessor,
> is the resulting code the naive translation, like in,
>
>  [(x, y) | x <- xs, even xs, y <- ys]
>
> =3D>
>
>  List.flatten (
>   List.map (fun x -> List.map (fun y -> (x, y)) ys) (List.filter even xs)
>  )
>
> or is there an optimization in order to avoid appends and minimize the
> number of cons?


FYI, my (old) syntax extension=B9 for camlp4 <=3D 3.09 expands list
comprehensions to a combination of folds:

  [+ (x, y) | x <- xs | when even x | y <- ys]

=3D>

  List.fold_right
    (fun x __acc__ ->
       if even x then
         List.fold_right (fun y __acc__ -> (x, y) :: __acc__) ys __acc__
       else __acc__)
    xs []

Less cons operations, but it's not tail recursive.

[1] http://oandrieu.nerim.net/ocaml/#pa_compr
--=20
  Olivier