Excerpts from christian.sternagel's message of Tue Jan 22 14:33:55 +0100 2008:
> > > How about the transformation from Chapter 7 of [1] (by Philip Wadler)?
> > > It should be similar to the `pseudo code':
> > > 
> > > type expr = ...;;
> > > type patt = ...;;
> > > type qualifier = Gen of patt * expr | Filt of expr;;
> > > type compr = (expr * qualifier list);;
> > > let rec expr = function
> > >  | ...
> > >  | (e, qs) -> transform [] (e, qs)
> > >  | ...
> > > and transform l = function
> > >  | (e, []) -> expr e :: expr l
> > >  | (e, Filt f :: qs) -> if expr f then transform l (e, qs) else expr l
> > >  | (e, Gen (p, l1) :: qs) ->
> > >   let rec h = function
> > >    | [] -> expr l
> > >    | u :: us -> (function p -> transform (h us) (e, qs) | _ -> h us) u
> > >   in h (expr l1)
> > > ;;
> > > 
> > > (* where h, u, us are fresh variables not occurring in e, l1, l, or qs *)
> > > 
> > > Sorry I'm not yet familiar with camlp4 grammar extensions, but of course
> > > above code would make use of them otherwise.
> > 
> > Yes this approach can be integrated with a camlp4 extension.
> > 
> > > It is stated in [1] that the resulting code is optimal in that it
> > > performs the minimum number of cons operations.
> > 
> > Nice.
> > 
> > > And I did ignore the hint that fresh variables make things
> > > complicated :).
> > 
> > Yes it can...
> > 
> > Best regards,
> > 
> > -- 
> > Nicolas Pouillard aka Ertai
> > 
> I deduce that there is no standard way of introducing
> `fresh' (w.r.t. the abstract syntax tree) variables
> within a camlp4 syntax extension? Wouldn't that be nice? =)
> 

That  would be nice, but doing it cleanly would require a large amount of work
and user visible changes.

-- 
Nicolas Pouillard aka Ertai