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From: "Nicolas Pouillard" <nicolas.pouillard@gmail.com>
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Subject: Re: [Caml-list] camlp4
To: caml-list <caml-list@yquem.inria.fr>
References: <1200676132.4790dd24bfca9@web-mail2.uibk.ac.at>
In-Reply-To: <1200676132.4790dd24bfca9@web-mail2.uibk.ac.at>
Date: Fri, 18 Jan 2008 19:56:47 +0100
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X-Spam: no; 0.00; camlp:01 camlp:01 flatten:01 appends:01 syntax:01 qualifiers:01 preprocessor:01 caml-list:01 optimization:03 concat:03 concat:03 naive:03 cons:04 translate:04 aka:04 

Excerpts from christian.sternagel's message of Fri Jan 18 18:08:52 +0100 2008:
> When using `camlp4o -parser Camlp4ListComprehension' as preprocessor,
> is the resulting code the naive translation, like in,
> 
>  [(x, y) | x <- xs, even xs, y <- ys]
> 
> =>
> 
>  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?

There is only a few very local optimizations.

However  you  can  answer  to your question by asking camlp4 to translate your
code and pretty-print the result.

$ camlp4o -parser Camlp4ListComprehension -str '[(x, y) | x <- xs; even xs; y <- ys]'
List.concat
  (List.map (fun x -> List.map (fun y -> (x, y)) ys)
     (List.filter (fun x -> even xs) xs))

As you can see List.concat is still there.

Note  also  that  the  syntax is to use `;' to separate qualifiers (generators
and filters) instead of `,' as your example.

Best regards,

-- 
Nicolas Pouillard aka Ertai