Re: [Caml-list] camlp5/revised syntax questions

[blue storm <bluestorm.dylc@gmail.com>]

Disclaimer : I have learned camlp4 from the ocaml distribution >=
3.10, wich is different from camlp5 : take my words with a grain of
salt.

On Wed, Oct 7, 2009 at 6:20 PM, Aaron Bohannon <bohannon@cis.upenn.edu> wrote:
> >From reading the camlp5 documentation, I've managed to write a syntax
> extension that adds a new expression starting with a distinct keyword,
> and it seems to work fine.  However, if I want to experiment with
> infix notations, things get a little trickier.  I need to specify it's
> precedence and associativity, of course.
>
> So, there is a list of syntactic structures on this page:
> http://pauillac.inria.fr/~ddr/camlp5/doc/htmlc/ast_transi.html
>
> 1) Where can I find the "level names" for each of these syntactic
> constructs, for use with BEFORE, LIKE, etc?  Is that what the
> "Comment" column is for?

The different "level names" are not absolute among all camlp4 grammars
: they're a property of each grammar rule of each grammar. If you want
to "modify" a specific grammar (that is, EXTEND it), you must check
the different levels available in the definition. See the files
meta/pa_r.ml and etc/pa_o.ml in the camlp5 source tree for the
definition of the revised and classical syntax, respectively. Luckily,
the expr rules (the one defining ocaml expressions, wich you seem
interested in) of both syntax mostly share the same levels (the
revised syntax has an additional "where" level for example, but
they're otherwise mostly the same).

There is no explicitely "precedence" in camlp4 parlance, you must use
the levels instead : each precedence level of infix operators has it
own level, usually named after the most representative infix operator
of the level : ':=', '||', '&&', '<'...

> 2) I am confused by the fact that this is a list is for the revised
> syntax.  I think most people (including me) want to modify the
> original syntax.  e.g., imagine that I want to modify the record
> update operator "<-" in the original syntax---I need to refer to it
> somehow, but it doesn't even appear in the list.

While concrete syntaxes for revised and classical syntax are
different, the abstract syntax tree is the same. Camlp4 quotations
works by replacing (using camlp4) the quotation you wrote by the
concrete ocaml AST representation. A nice side-effet of this is that
you can use quotations "in the revised syntax" (the code inside
quotations use the revised syntax) when writing an extension for the
classical syntax. Eg. if you parse "for i = 0 to 10 step 2 do ...
done" and you want to generate the OCaml AST corresponding to "for i =
0 to 10/2 do let i = i * 2 in ... done", you can write <:expr< for i =
0 to 10 / 2 do { let i = i * 2 in ... } >> , it will generate the
corresponding AST and be printed (after processing the source using
the extenion) in whatever syntax the user of your extension is using
(probably the classical one).

This way (using revised syntax inside the quotation of your
extension), your can stay consistent with the camlp5 documentation
(wich describes the quotation in the revised syntax). camlp4 >= 3.10
also has quotations in the classical syntax, but I wouldn't recommend
using them : revised syntax is a less ambiguous syntax wich makes
those things easier.

In your specific case, you can parse whatever syntax you want using
the "<-" operator, then output the corresponding AST using a quotation
in the revised syntax, that is <:expr< a := b >> (instead of "a <-
b"). For a reference, see the related rules in etc/pa_o.ml :

  | ":=" NONA
    [ e1 = SELF; ":="; e2 = expr LEVEL "expr1" ->
      <:expr< $e1$.val := $e2$ >>
    | e1 = SELF; "<-"; e2 = expr LEVEL "expr1" ->
      <:expr< $e1$ := $e2$ >> ]