ocaml / ocaml/dune

ppx is skipped when `(modes byte)`

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preprocessing
Dominant language
OCaml
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Description

Expected Behavior

I define a ppx which is expected to compile only as a bytecode library:

(library
 (name my_ppx)
 ...
 (kind ppx_rewriter)
 (modes byte))

I expect this to be applied to my code which depends on my_ppx as a preprocessor

...
  (preprocessor (pps my_ppx))
...

Actual Behavior

The ppx is not applied to the code and extension points remain. This is not the case if I remove the (modes byte).

Reproduction

See above

Specifications

  • Version of dune (output of dune --version): 3.12.1
  • Version of ocaml (output of ocamlc --version): 4.14.1
  • Operating system (distribution and version): macOS Big Sur

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the minimal Dune reproduction in the issue using a bytecode-only ppx_rewriter and a consumer with (preprocessor (pps my_ppx)). Compare the build behavior with and without (modes byte); done means the ppx is applied and extension points no longer remain in the generated code.

Written by the indexing model from the issue text.

Assessment

Tech stack
ocaml
Domain
build-system
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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