In short...

As you've seen it the output of the functor is not used by the
Register module. However that's not the only possible usage of an
extension functor.

If you look at Camlp4Parsers/Camlp4OCamlOriginalQuotationExpander.ml

open PreCast;
let module Gram = MakeGram Lexer in
let module M1 = OCamlInitSyntax.Make Ast Gram Quotation in
let module M2 = Camlp4OCamlRevisedParser.Make M1 in
let module M3 = Camlp4OCamlParser.Make M2 in
let module M3 = Camlp4QuotationCommon.Make M3 Syntax.AntiquotSyntax in ();

Here syntax extensions are plugged together to make a new grammar. If
your extension is functorized you can extend that language easily.

However one can argue that a signature like <<Syntax -> sig end>> would suffice.

Indeed I wanted to make it more functional, but I failed it seems that
we need first class modules.

Finally it will be perhaps simpler to switch to a signature like
<<Syntax -> sig end>>

On 7/17/07, Bruno De Fraine <Bruno.De.Fraine@vub.ac.be> wrote:
> Hello,
>
> Here's a minimal syntax extension in the functor-style, which seems
> to be preferred style with the new camlp4. It defines a PI constant
> which can be used inside expressions:
>
>    module Id = struct let name = "pi" and version = "3.14" end
>
>    open Camlp4.Sig
>
>    module Pi(Syntax : Camlp4Syntax) = struct
>      include Syntax
>      EXTEND Gram
>        expr: LEVEL "simple"
>        [[ "PI" -> <:expr< $flo:"3.14159265358979312"$ >> ]];
>      END
>    end
>
>    let module M = Camlp4.Register.OCamlSyntaxExtension(Id)(Pi) in ()
>
> As mentioned in the documentation, an extension (such as Pi) must be
> functor: Camlp4Syntax -> Camlp4Syntax, and
> Register.OCamlSyntaxExtension requires this type. This is why I have
> to "include" (instead of "open") the original syntax: to produce a
> valid output syntax. My question is: how is this output syntax ever
> used? (Note that the EXTEND-statement does not make any structural
> changes to the syntax module, just dynamic changes as a side-effect
> upon functor application.)
>
> To put all my cards on the table: I believe the output syntax is
> never used. For example, you can sabotage one of the main grammar
> entries by finishing the definition of Pi with:
>
>    let top_phrase : Ast.str_item option Gram.Entry.t = Obj.magic 0
>
> And the extension keeps working all the same from the toplevel. In
> fact, a look at the code of OCamlSyntaxExtension in Register.ml
> confirms it is never used:
>
>    module OCamlSyntaxExtension
>      (Id : Sig.Id) (Maker : functor (Syn : Sig.Camlp4Syntax) ->
> Sig.Camlp4Syntax) =
>    struct
>      declare_dyn_module Id.name (fun _ -> let module M = Maker Syntax
> in ());
>    end;
>
> The output syntax M is thrown away, i.e. the syntax extension relies
> entirely on side-effects of the functor application. I think the type
> required for a syntax extension could just as well have been a
> functor that return an empty module: Camlp4Syntax -> sig end
>
> Why bother making this remark if you can just include the original
> syntax at the beginning and it works? I believe there are two
> important reasons. The first is didactical: the signature
> Camlp4Syntax -> Camlp4Syntax suggests that the syntax extension works
> by structurally transforming one syntax into another, while this is
> not what is going on. This situation makes the workings of camlp4 all
> the more difficult to understand for novice (and perhaps seasoned)
> camlp4 developers. The second reason is practical: you can easily
> define something in your extension that clashes with a name from
> Camlp4Syntax (e.g. "expr"), and then the compiler will complain if
> the types do not agree. You can assure you export the exact original
> definitions by putting "include Syntax" at the end of the extension
> instead of the beginning, but then you still need an "open Syntax" at
> the beginning to have the useful modules (like Ast, Gram, etc.)
> available. All of this is an annoying redundant idiom given that the
> output Syntax is not used.
>
> Regards,
> Bruno
>
>
> --
> Bruno De Fraine
> Vrije Universiteit Brussel
> Faculty of Applied Sciences, DINF - SSEL
> Room 4K208, Pleinlaan 2, B-1050 Brussels
> tel: +32 (0)2 629 29 75
> fax: +32 (0)2 629 28 70
> e-mail: Bruno.De.Fraine@vub.ac.be
>
>
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-- 
Nicolas Pouillard