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Date: -- (:)
From: Bardur Arantsson <list-caml-list@s...>
Subject: Re: [Caml-list] Camlp4 with traditional syntax
On Mon, Feb 21, 2005 at 04:22:24PM +0100, Olivier Andrieu wrote:

>  > Bardur Arantsson [Mon, 21 Feb 2005]:
>  > On Mon, Feb 21, 2005 at 01:28:25PM +0100, Alex Baretta wrote:

>  > > Hendrik Tews wrote:
>  > > >Alex Baretta <alex@barettadeit.com> writes:

>  > > >   There is one more issue with Camlp4: it does not allow for quotations
>  > > >   to expand to generic syntactic elements. Often, I use quotations which
>  > > >   expand to module definitions. I had to implement my own quotation
>  > > >   expander, bypassing the limitations of Camlp4 to achieve this.

>  > > >I don't quite understand, what's wrong with 

>  > > >let me = <:module_expr< struct $ list of module def's $ end >>
>  > > >in
>  > > >  <:str_item< module $some_name$ = $me$ >> 

>  > > We use quotation expanders to embed completely different
>  > > languages, such as SQL, within Ocaml code. Specifically, the SQL
>  > > quotation expander compiles SQL code to an Ocaml module. CamlP4
>  > > signals an error because quotations are only meant to be used as
>  > > expressions or as patterns, IIRC.

>  > IIRC quotations can expand to arbitrary ASTs. Only the point of
>  > *use* (ie. substitution) determines which types of ASTs will be
>  > accepted.

> No, quotations can expand to only expressions or patterns. See the
> type of a quotation expander in quotation.mli :

>   type expander =
>   | ExStr of (bool -> string -> string)
>   | ExAst of ((string -> MLast.expr) * (string -> MLast.patt))

> As Alex mentionned, ocpp expands quotations everywhere (but only deals
> with string-expanding quotations, not the AST ones), so it can be used
> to generate structure items (module elements).

You're right. I got confused by the distinction between user-defined and
pre-defined quotation expanders.

>  > Of course, if you're generating things like module interfaces and
>  > implementations, you'll need to generate them side by side since
>  > there is no "combined module interface+implementation" AST node
>  > type.

> The camlp4 AST does have this kind of nodes, as Martin mentionned.
> You can use :

> # fun a b -> <:str_item< declare $a$ ; $b$ ; end >> ;;
> - : MLast.str_item -> MLast.str_item -> MLast.str_item = <fun>

> or :

> # fun l -> <:str_item< declare $list:l$ end >> ;;
> - : MLast.str_item list -> MLast.str_item = <fun>

Uh, yes, you use <str_item:<...> to generate module implementations, but 
when I looked last you couldn't use MLast.sig_item and MLast.str_item
interchangably, so for instance, you couldn't ask pr_(o|r) to
pretty-print a str_item as a module interface. Instead you had to also
generate a sig_item and pretty-print that. (For good reason, you don't
necessarily want to have everything inside the module visible from the
outside...). That's all I was referring to by generating them "side by
side".

-- 
Bardur Arantsson
<bardur@imada.sdu.dk>
<bardur@scientician.net>

- It might look like I'm standing motionless, but I'm actively
waiting for my problems to go away.
                                                      Scott Adams