Just out of curiosity, what does "vistors" do? If it resembles the usual
Visitor Pattern you might find my ppx_deriving_morphism useful:

https://github.com/choeger/ppx_deriving_morphism/

(either as a starting point or inspiration or as something to obsolete).

Am 04.01.2017 um 14:08 schrieb François Pottier:
> 
> Hello all,
> 
> I am currently in the process of writing a ppx_deriving plugin, called
> "visitors". Overall, this has been a pleasant experience; a few hundred
> lines
> of code have been sufficient to obtain nontrivial results.
> 
> In normal use, the user writes something like this:
> 
>   type foo =
>     Bar | Baz
>     [@@deriving visitors]
> 
> and some generated code is inserted just after the definition of the
> type foo.
> 
> However, I have reached a situation where the generated code cannot be
> placed
> just after the type definition. That is, I need to allow user-written
> code to
> appear after the type definition and before the generated code.
> 
> For instance, this user-written code could be a declaration of a global
> variable "x", whose type is "foo ref", and which the generated code
> uses. The
> declaration of "x" must appear after the definition of the type "foo",
> because
> the type of "x" mentions "foo". And the declaration of "x" must appear
> before
> the generated code, because the generated code (intentionally) refers to
> "x".
> 
> I am imagining that perhaps the user could write something like this:
> 
>   type foo =
>     Bar | Baz
>     [@@deriving visitors { delayed = true }
> 
>   let x : foo option ref =
>     ref None
> 
>   [@@visitors]
> 
> The effect of the flag { delayed = true } would be to store the
> generated code
> somewhere in memory (instead of emitting right away), and the effect of the
> floating attribute [@@visitors] would be to fetch that code from memory and
> emit it.
> 
> To me, this seems somewhat ugly, but workable. Does ppx_deriving offer a
> better approach? Does anyone have a better suggestion? Comments are
> appreciated.
> 
> Best regards,
> 
> -- 
> François Pottier
> francois.pottier@inria.fr
> http://gallium.inria.fr/~fpottier/
> 


-- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de