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Date: Mon, 23 Aug 2004 01:13:52 -0400 (EDT)
From: Michael Alexander Hamburg <hamburg@fas.harvard.edu>
To: caml-list@pauillac.inria.fr
Subject: [Caml-list] camlp4 and class values
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I'm trying to write a simple game in ocaml, largely to give me an excuse
to learn more of the language.  To deal with rule-changing effects, a
large fraction of the methods in the game will have to be more or less
mutable, although they will be mutated in a specific, nontrival way.

I've created a module, temporarily called Stack, which implements the kind
of mutation I want, so now there's the issue of define the methods.  For
now, the implementation goes thusly:

class test = object (self)
  val onFoo : (test -> int -> int) Stack.t =
    Stack.create (fun _ _ -> 0)

  method foo x = Stack.apply onFoo self x

  method addFoo f = Stack.append onFoo f
end

And this sort of thing works, but it would be nice if I didn't have to
write these three things for all the extensible methods, which is quite a
lot of methods.  So I've been trying to write a camlp4 preprocessor, which
would take

class test = object (self)
  extmethod foo (x:int) = 0
end

or the like, and turn it into the above code.  I haven't been able to get
anything to work, though.  I don't really know camlp4, although not for
lack of trying (I've spent several hours working through its
documentation).  The best I've been able to do is something like:

#load "pa_extend.cmo";;
#load "q_MLast.cmo";;

open Pcaml
open MLast

let addName n = "add_" ^ n
let onName n = "on_" ^ n

let o2b = function
  Some _ -> true
| None -> false

let genfunc loc params e =
  <:expr< Stack.apply (fun self $params$ -> $e$)  >>

let genval loc l params e =
 <:class_str_item< value $lid:onName l$ = $genfunc loc params e$ >>

let genmeth loc pf l topt params =
 <:class_str_item< method $opt:o2b pf$ $l$ $opt:topt$ $patt:params$ =
   Stack.apply $lid:onName l$ self $params$>>

let genadd loc pf l =
 <:class_str_item< method $opt:o2b pf$ $addName l$ f = Stack.append
$lid:onName l$ f >>


EXTEND
  class_str_item : LAST
  [[ "extmethod"; pf = OPT "private"; l = LIDENT; topt = OPT ctyp; params
= LIST0 patt; "="; e = expr ->
       let meth = genmeth loc pf l topt params
       and add = genadd loc pf l
       and v = genval loc l params e in
	 <:class_str_item< declare $v$; $meth$; $add$; end >>
  ]];
END;;

which raises:

File "extmethod.ml", line 22, characters 52-65:
While expanding quotation "class_str_item":
Parse error: [fun_binding] expected (in [class structure item])
Uncaught exception: Pcaml.Qerror("class_str_item", 1, _)
Preprocessor error

The closest thing I've been able to find to documentation on fun_bindings
is the camlp4 source, which does a much more direct translation.

How do I write such an extenision? OR is there some reason that it's not
possible?

Thanks for your time,

Mike Hamburg

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