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Date: Tue, 25 Dec 2007 18:36:27 +0100
From: "blue storm" <bluestorm.dylc@gmail.com>
To: caml-list@yquem.inria.fr
Subject: [camlp4 extension] pa_refutable : request for comments
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X-Spam: no; 0.00; camlp:01 camlp:01 syntax:01 foo:01 runtime:01 failwith:01 expr:01 expr:01 subtleties:01 storm:98 rec:01 rec:01 parsing:01 patt:01 patt:01 

Hi,

I just created my first "serious" camlp4 (3.10) extension. I'm looking
for comments, and have some questions too.

The extension (to the classical syntax) enables an explicit use of
non-irrefutable pattern matching in "let" declarations :
let refutable hd::tl = ...
let refutable [a; b; c] = List.map foo ['a'; 'b'; 'c']
let rec refutable func (Some thing) = ...

The code is available here : http://bluestorm.info/tmp/pa_refutable.ml.html
(side question : are 80 lines of code short enough to be included in
my message ? what is the mailing-list recommended behaviour here ?)

The actual error-reporting is rather naive : the extension generate a
string containing the location of the refutable pattern in the
original source, wich is raised at runtime using "failwith".. Is there
a more elegant way to do that ?

There are 6 repetitive lines of code in my refutable_mono function :
    let binds_patt binds _loc =
      let patt_of_id (id, _loc) = <:patt< $lid:id$ >> in
      Ast.PaTup _loc (Ast.paCom_of_list (List.map patt_of_id binds)) in
    let binds_expr binds _loc =
      let expr_of_id (id, _loc) = <:expr< $lid:id$ >> in
      Ast.ExTup _loc (Ast.exCom_of_list (List.map expr_of_id binds)) in
Is there an better way to do this ?

Lastly, i had to duplicate work in the grammar-modification part,
because i didn't manage to use the "opt_rec" rule. I'm not aware of
the subtleties of camlp4 parsing; is there another way ?