camlp4: processing lists

camlp4: processing lists

[Hugo Ferreira]

Hello,

I have sent this post to the new ocaml-developer list but am resending 
it here in the hopes that someone can help me. I have come across a 
weirdness in camlp4. Maybe someone can explain this puzzle to me.

I have the following extension:

let expand_term_list loc l =
            let nl = <:expr< [] >>                                      in
            let nl = List.fold_right (fun h t ->
                    <:expr< [$h$::$t$] >>
                    ) l nl                              in
            <:expr<
            do {
                   List.iter (fun i -> print_endline i) $nl$
             }
            >>

EXTEND
    Pcaml.expr: [
      [
      "|" ; "[" ; l = LIST0 Pcaml.expr  SEP ";" ; "]" ; "|"->
                   expand_term_list loc l ]  ];
END;;

I compile the source below with the camlp4 extension above:

....

    let t1 =
      List.iter (fun i -> print_endline i) ["1"; "f(X,Y)"; "g(X,Y)"; "4"]
    in
    ()

And the execution gives me:

4

If I generate the original and revised syntax output using the extension
above using for example:

camlp4o -I . pr_o.cmo pa_$(NAME).cmo $(PROG).ml -o $(PROG).ppo
camlp4o -I . pr_r.cmo pa_$(NAME).cmo $(PROG).ml -o $(PROG).ppr

I get for $(PROG).ppo

...
    let t1 =
      List.iter (fun i -> print_endline i) ["1"; "f(X,Y)"; "g(X,Y)"; "4"]
    in
    ()

which is what I expect for the original syntax... but for the revised
syntax $(PROG).ppr I get:

    let t1 =
      do {
        print_string "Not implemented for list ";
        List.iter (fun i -> print_endline i)
          [do { "1"; "f(X,Y)"; "g(X,Y)"; "4" }]
      }
    in
    ()

which ... to say the least is *not* what I expected. I initially I
though I had but one Pcaml.expr and not a list of those.  What seems
to be happening is that the revised syntax is used to generate the
code and this is wrong.

Can anyone tell me what mess I have done?

TIA,
Hugo Ferreira.

Re: [Caml-list] camlp4: processing lists

[Nicolas Pouillard]

On 3/20/07, Hugo Ferreira <hmf@inescporto.pt> wrote:
> Hello,
>
> I have sent this post to the new ocaml-developer list but am resending
> it here in the hopes that someone can help me. I have come across a
> weirdness in camlp4. Maybe someone can explain this puzzle to me.
>
> I have the following extension:
>
> let expand_term_list loc l =
>             let nl = <:expr< [] >>                                      in
>             let nl = List.fold_right (fun h t ->
>                     <:expr< [$h$::$t$] >>
>                     ) l nl                              in
>             <:expr<
>             do {
>                    List.iter (fun i -> print_endline i) $nl$
>              }
>             >>
>
> EXTEND
>     Pcaml.expr: [
>       [
>       "|" ; "[" ; l = LIST0 Pcaml.expr  SEP ";" ; "]" ; "|"->

Pcaml.expr is too greedy here. Since 1; 2 is a valid ocaml expression.
You need to choose another level ("expr1" in your case and "top" in 3.10).

>       "|" ; "[" ; l = LIST0 Pcaml.expr  LEVEL "expr1" SEP ";" ; "]" ; "|"->

>                    expand_term_list loc l ]  ];
> END;;
>

[...]

>
>     let t1 =
>       do {
>         print_string "Not implemented for list ";

Here you reached a bug (fixed for 3.10)

>         List.iter (fun i -> print_endline i)
>           [do { "1"; "f(X,Y)"; "g(X,Y)"; "4" }]
>       }
>     in
>     ()
>
> which ... to say the least is *not* what I expected. I initially I
> though I had but one Pcaml.expr and not a list of those.  What seems
> to be happening is that the revised syntax is used to generate the
> code and this is wrong.

The in revised 1; 2 is not a valid expression you need to add do { ...
} thus by printing it in the revised syntax you see that was not what
you want.
BTW it was a bug in camlp4 to print [1; 2] where it was [(1; 2)].

Hope this helps,

-- 
Nicolas Pouillard

Re: [Caml-list] camlp4: processing lists

[Hugo Ferreira]

Hi,

Nicolas Pouillard wrote:
> On 3/20/07, Hugo Ferreira <hmf@inescporto.pt> wrote:
>> Hello,
>>
> 
> Here you reached a bug (fixed for 3.10)
> 
>>         List.iter (fun i -> print_endline i)
>>           [do { "1"; "f(X,Y)"; "g(X,Y)"; "4" }]
>>       }
>>     in
>>     ()
>>

So I take it that I either wait for 3.10 or circumvent this problem with 
  a non-revised syntax print.

> 
> The in revised 1; 2 is not a valid expression you need to add do { ...
> } thus by printing it in the revised syntax you see that was not what
> you want.
> BTW it was a bug in camlp4 to print [1; 2] where it was [(1; 2)].
> 
> Hope this helps,
> 

It has indeed.


Thank you,
Hugo Ferreira.