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To: caml-list@inria.fr
Subject: Re: [Caml-list] A question about Camlp4
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From: Remi Vanicat <vanicat@labri.u-bordeaux.fr>
Date: Thu, 03 Apr 2003 11:40:59 +0200
In-Reply-To: <3E8BFE27.9040602@inrialpes.fr> (Frederic Tronel's message of
 "Thu, 03 Apr 2003 11:25:59 +0200")
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Frederic Tronel <Frederic.Tronel@inrialpes.fr> writes:

> Hi list,
>
>
> I'm using camlp4 in order to embed a scripting language within my caml
> programs.
> I've already done this one year ago for a subset of Lotos verification
> language, and I'm extending this work.
>
> I'm facing a difficulty. Here is a canonical example of the problem:
>
> EXTEND:
>     rule1: ..... ->    <:expr< >>   (* A rule that generate a ML AST *)
>     rule2:  OPT a = rule1 ..... ->
> 	let b = match a with
> 		None -> <:expr<None>>
> 		Some x -> <:expr<Some $x>>
> 	in
> 	<:expr< ATypeConstructor  $b$>>
> ...
> END	
>
> I have to do this small trick for each optionnal symbol in each rule,
> (this is also true for list meta symbols introduced by LIST0, LIST1
> ...).
> It would greatly improve readability of my program, if I could define
> a small function outside from the grammar extension which would do:
>
> optionToAst o =
> match o with
>   None -> <:expr<None>>
>   Some x -> <:expr<Some $x>>
>
> However if I do this the compiler complains about unbound value "loc".
> Thanks for your help.


there is an "hidden" loc "argument" to expr quotation, and this loc
variable is defined in the action.

let optionToAst loc o =
match o with
| None -> <:expr<None>>
| Some x -> <:expr<Some $x>>

then you could call "optionToAst loc a" and it will work...

-- 
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat

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