[Caml-list] camlp4 and class values

[Caml-list] camlp4 and class values

[Michael Alexander Hamburg]

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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Re: [Caml-list] camlp4 and class values

[Dmitry Lomov]

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
> 
> -------------------
> To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
> Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> 


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

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