How to use preprocessing in obuild ?

[douglarek]

Take filedslib for example.

name: hello
version: 0.1.0
synopsis: A demo.
obuild-ver: 1

executable hello
  main: main.ml
  src-dir: src
  build-deps: core

module Logon = struct
  type t =
    {
      session_id: string;
      time: Time.t;
      user: string;
      credentials: string;
    } with fields (* here *)
end

[jeromemaloberti]

AFAICT fieldslib seems to be only a library, you would need ppx_fields_conv (for ppx) or pa_fields_conv (for camlp4) to use it as an extension. The "modern" way would be the ppx version, just add ppx_fields_conv to the build-deps of your exe/lib and it should preprocess the file and compile it from obuild 0.1.8. If it doesn't please provide your test and open a ticket, I will fix the bug. It is also possible with camlp4 (a little bit more complicated but not much), however I couldn't install pa_fields_conv with ocaml 4.02.3 ...

[douglarek]

@jeromemaloberti thanks for your quick reply;

this is my project :https://gist.github.com/douglarek/9e7915e847245b236890179af3d49209;

It failed:

File "src/main.ml", line 32, characters 6-10:
Error: Syntax error

[jeromemaloberti]

Thank you. It is indeed a bug, I forgot to use the preprocessor when calling ocamldep to get the dependencies. I will fix it tonight.

[jeromemaloberti]

Actually, I got it wrong. The problem is that you're using the camlp4 extension with ppx. You must replace "with fields" with "[@@deriving fields]" and it will work. You can see the example: https://github.com/janestreet/ppx_fields_conv/blob/master/example/test.ml

[douglarek]

Thanks, I am a newbie to OCaml, but how can I use with fields rather than [@@devering fields], I am reading Real world OCaml book.

[rgrinberg]

@douglarek Why can't you just do [@@deriving fields]? It should generate the same code for you.

[douglarek]

@rgrinberg this is an example in book Real world OCaml, but it did not work with obuild

[rgrinberg]

@douglarek post your project in a git repo somewhere

[douglarek]

@rgrinberg Please follow https://gist.github.com/douglarek/9e7915e847245b236890179af3d49209

[jeromemaloberti]

Sorry for the late answer. You can use pa_fields_conv ("with fields") by adding pa_fields_conv to the build-deps (you need to install it, of course) and by adding: pp: camlp4o in your target description. So: executable hello main: main.ml src-dir: src pp: camlp4o build-deps: core, pa_fields_conv

[jeromemaloberti]

However, pa_fields_conv has been tagged deprecated, so maybe you should think about using ppx_fields_conv and replace "with fields" with "[@@deriving fields]" ...

[douglarek]

@jeromemaloberti But I just tried [@@deriving fields], it works without ppx_fields_conv preprocessing, maybe it is part of core. And my OCaml version is 4.02.

[jeromemaloberti]

Yes, and no. ppx are annotations, the code compile without the preprocessing, but no code was generated, so if you try to use the supposedly generated code, it will fail.

[douglarek]

@jeromemaloberti thanks, it is very kind of you to answer so lubberly problem.