[Camlp4 3.10] The rosetta stone (Was: lists without $list:...$)

["Nicolas Pouillard" <nicolas.pouillard@gmail.com>]

On 3/24/07, Martin Jambon <martin.jambon@ens-lyon.org> wrote:
>
> I would like you or anyone knowledgeable to translate one significant
> syntax extension. You know, some kind of Rosetta stone. You can take
> Markus' sexp syntax extension or ioxml if you prefer, it won't be much
> different.

Here is your rosetta stone :)

I translated pa_json_static.ml.

To highlight a little the changes in the AST,
I want to show an input snippet and two translations:

For the old camlp4 it was:

| Object l ->
   let ml = List.map (fun x -> (x.field_caml_name,
                                convert x.field_type)) l in
   <:ctyp< < $list:ml$ > >>

The type of `ml' is (string * ctyp) list. The concept of quotations is
to get concrete syntax for abstract terms and then avoid to learn all
constructors and types. Alas for some of them you have to know the type.
In the new version you can express any term (except one) by concrete syntax.

The closest version is:

| Object l ->
   let ml = List.map (fun x ->
     <:ctyp< $lid:x.field_caml_name$ : $convert x.field_type$ >>) l in
   <:ctyp< < $list:ml$ > >>

Here one doesn't know if methods declarations are a pair or something else and
we don't care. Since one knows the syntax << method_name : method_type >>.
At this place the list antiquotation $list:ml$ is a sugar for
$Ast.tySem_of_list ml$.

By changing a little more the code one can use something closer to the
object syntax.

| Object l ->
   let ml = List.fold_right (fun x acc ->
     <:ctyp< $lid:x.field_caml_name$ : $convert x.field_type$ ; $acc$ >>)
     l <:ctyp<>> in
   <:ctyp< < $ml$ > >>

The general syntax of object types (omiting `..' for the row variable) is
< meth1 : type1 ; ... ; methN : typeN > then one can avoid to construct a list
(since map is a fold_right with `::') and then call a function that destruct it.

Another thing to point out is the use of the nil type <:ctyp<>> that is quite
useful to start the folding.

In this translation I used both styles depending of the context.

The patch:
  http://gallium.inria.fr/~pouillar/pub/camlp4/rosetta/pa_json_static/pa_json_static.patch

The new version (compiles with camlp4orf):
  http://gallium.inria.fr/~pouillar/pub/camlp4/rosetta/pa_json_static/pa_json_static.ml

Cheers,

--
Nicolas Pouillard