Camlp4 mysteries (was Re: On language extensions (was Re: [Caml-list] global record))

[Martin Jambon <martin1977@laposte.net>]

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On Thu, 20 Jul 2006, Bünzli Daniel wrote:

> I'm sorry to say that but I regard (maybe out of ignorance) camlp4 as very 
> low level and brittle tool (not to say hack). An obvious proof of this to me 
> is the composition problem which is according to previous messages is not 
> solved by camlp4.

In theory the system of quotations allows you to use as many quotation 
expanders as you want. Quotations are well delimited and do not interfere 
with each other, but look a bit ugly and are restricted to exprs and 
patts.

Otherwise it's possible to define well-disciplined syntax extensions.
For example, if each new syntax construct (new rule) is forced to start 
with a unique, registered keyword and end with "end", then 
different syntax extensions that follow this rule should play well 
together. 
Deleting or rewriting existing rules would of course be forbidden.
And tools like Declare_once [1] should become builtins.

So if you take my favorite syntax extension (micmatch), you would need to 
create a new keyword, let's say "mm":

   match "Hello World!" with
       / "Hello"~ blank+ (alnum+ as user) / -> Some user
     | _ -> None

could become

   mm match "Hello World!" with
      mm "Hello"~ blank+ (alnum+ as user) end -> Some user
    | _ -> None
   end


To the OCaml development team:

Would such a convention make sense?
Could there be special restricted EXTEND statement that only accepts that 
kind of rules, and registers the keywords to avoid unexpected clashes 
between extensions that try to use the same leading keyword?

It would be really nice to have official guidelines on how to develop 
clean syntax extensions, if not automatic enforcement.

That's all I can say, I don't have time to spend playing with the 
new Camlp4 or trying to understand what MetaOCaml is about.
Any official word about the metaprogramming issues in OCaml would be 
appreciated!
I am sorry to say that, but there is way too much mystery floating around 
Camlp4. A web page that presents a roadmap for the development of OCaml 
language and tools including Camlp4 would be much appreciated.


Martin


[1] Declare_once: look for it at http://www.yl.is.s.u-tokyo.ac.jp/~oiwa/en_US.ISO-8859-1/pub/caml/regexp-pp-1.0.0/README.match-regexp

--
Martin Jambon, PhD
http://martin.jambon.free.fr