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Organization: Flying Frog Consultancy Ltd.
To: "Nicolas Pouillard" <nicolas.pouillard@gmail.com>,
	caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Re: Metaprogramming features
Date: Tue, 7 Oct 2008 16:36:43 +0100
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References: <884069.59268.qm@web54601.mail.re2.yahoo.com> <200810070117.13329.jon@ffconsultancy.com> <1223383737-sup-3904@ausone.local>
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From: Jon Harrop <jonathandeanharrop@googlemail.com>
X-Spam: no; 0.00; discrepancy:01 printf:01 printf:01 metaocaml:01 metaocaml:01 ocaml:01 faithful:98 frog:98 unbound:01 closures:01 wrote:01 caml-list:01 formulation:01 expression:02 defined:02 

On Tuesday 07 October 2008 13:49:29 you wrote:
> Excerpts from jonathandeanharrop's message of Tue Oct 07 02:17:13 +0200 
2008:
> > Surely the discrepancy is because the translation should be:
> >
> >   let npower7 =
> >     let e = printf "power\n"; npower 7 (fun () -> x) in
> >     (fun () -> fun x -> e ()) ();;
>
> Hum "unbound value x" :)

Oh yeah. :-)

I hadn't noticed that you can do:

# .< fun x -> .~ .<x>. >.;;
- : ('a, 'b -> 'b) code = .<fun x_1 -> x_1>.

Which seems to be the source of my confusion. So "x" is bound inside the 
escaped code even though it was defined in the "fun x ->" of the bracketed 
code.

You are not allowed to do:

# .< fun x -> .~ x >.;;
Wrong level: variable bound at level 1 and used at level 0

That means a faithful translation into closures must capture such bound 
variables which will, in turn, defer the expression that does the 
constructing:

  let e x = printf "power\n"; npower 7 (fun () -> x)

and that now fails to incur side-effects at the same time as the MetaOCaml 
program.

So the translation from MetaOCaml to OCaml is not obvious at all.

This begs the question: could another formulation of MetaOCaml avoid these 
complications without losing expressiveness?

-- 
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e