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Date: -- (:)
From: Dmitry Lomov <dmitry.lomov@j...>
Subject: Re: [Caml-list] camlp4 and class values
Michael,

please see my inline answer.

Michael Alexander Hamburg wrote:

>  [skip]
> #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 >>

Quotations has a very strict syntax. In particular, quotation
<:class_str_item< declare ...  >>, which is a way to add multiple 
declarations to a class body, expects a list of class_str_item's.
If you just write some code in revised syntax, it will construct the 
list automatically, but it cannot construct the list from 
antiquotations. So you have to do it yourself:
  let csil = [v; meth; add] in
  <:class_str_item< declare $csil:csil$ end >>

Hope this helps,
Friendly,
Dmitry

>   ]];
> 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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-- 
Dmitry Lomov
JetBrains Inc.
http://www.jetbrains.com
"Develop With Pleasure!"

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