Adding both action and PPS in preprocess - Dune file

Learning
1

Hey everyone,

I am new to OCaml and wanted to know how can i add two preprocess in my dune file. Here’s the dune file and my use case is to add (pps ppx_deriving_yojson) in preprocess -

(library
 (name ml)
 (wrapped false)
 (preprocess
  (action
   (run %{bin:cppo} %{env:CPPO_FLAGS=} %{input-file})))
 (flags
  (:standard -w +a-4-42-40-41-44-45-9-48-67-70))
 (libraries ext js_parser))

(ocamllex lexer)
2

This is not possible at the moment. See Preprocess using both cppo and PPXs · Issue #171 · ocaml/dune · GitHub. There’s an implementation at feat: allow preprocessing action chains before `pps` by anmonteiro · Pull Request #7373 · ocaml/dune · GitHub but which has stalled.

3

Is there any workaround for this? My usecase is to add (pps ppx_deriving_yojson) so that i can use [@@deriving to_yojson] on various types defined.

Or this there any other alternative that i can use to get json from any OCaml type?

4

you can try to separate the parts that require cppo from the parts that require ppx.

5

How to do that? This dune file is of a package that i have vendored and don’t know much about their implementation.

7

No, I won’t join your meeting link. I can’t help you more. If this is an assignment, ask your supervisor. Thanks

8

Hello,

I need this to.

To separate, you may use a dune rule that will transform a file.tmpl.ml => file.ml and then run all pps you need while compiling file.ml. Here is an example:

 (rule
  (targets server.ml)
  (deps server.tmpl.ml)
  (locks /port)
  (enabled_if (= %{system} "linux"))
  (action (with-stdout-to %{targets} (run %{bin:chaml} %{deps}))))

(executable
  (name server)
  (modules db server)
  (flags (:standard -dsource -w -32))
  (preprocess (pps safeDb.ppx))
  (libraries simple_httpd safeDb Inverter))

Cheers,
Christophe

9

Folks, I think that everything should be already doable

(preprocess
 (pps
  someppx
   --
   -ppx-rewriter-switch 
   -pp  ; NB
   %{read:../config/which.cfg}/external_rewriter_executable.exe))