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