Incorrect interpretation of relative paths in FLG -ppx

[jberdine]

I am having trouble with merlin not being able to run ppxs built and configured with jbuilder. The jbuilder compilation works correctly.

jbuilder creates a ppx.exe under _build and adds a line such as the following to .merlin:

    FLG -ppx "_build/default/.ppx/ppx_mine/ppx.exe --as-ppx"

This seems fine, but looking at the merlin-log in emacs, I see:

    # 0.06 Mconfig - normalize
    {
      "ocaml": {
    ...
        "ppx": [ "/_build/default/.ppx/ppx_mine/ppx.exe --as-ppx" ],
    ...
      "merlin": {
        "flags_applied": [
          {
            "cwd": "/",
            "flags": [
              "-ppx",
              "./_build/default/.ppx/ppx_mine/ppx.exe --as-ppx"
            ]
          },
    ...
    }
    # 0.06 Mppx - changing_directory
    /Users/jjb/project/src
    # 0.07 Mppx - rewrite
    failed with Error while running external preprocessor
    Command line: /_build/default/.ppx/ppx_mine/ppx.exe --as-ppx '/var/folders/qd/00gsz33d7zj71k_dhp7bwv5cxm01z7/T/camlppxca635d' '/var/folders/qd/00gsz33d7zj71k_dhp7bwv5cxm01z7/T/camlppx5f4ce4' 1>/dev/null 2>/dev/null

Note the leading / on Command line:

If I edit .merlin to provide the absolute path in FLG -ppx, then everything works as expected.

Should relative paths in a .merlin file be interpreted relative to the directory containing the .merlin file, or something else? Is there an issue where the config is normalized before changing directory?

FWIW, I have merlin and ocaml-migrate-parsetree pinned to master, with a 4.05.0 switch.

[jberdine]

I don't know if it addressed the root cause, but after ocaml/dune#201, the .merlin files generated by jbuilder include absolute paths and no longer trigger this issue.

[alfa07]

It is still an issue as in our infra as we can't commit files with absolute paths into repo (e.g. it usually includes user's name)

[rgrinberg]

How come there's a need to commit .merlin files at all? Can't every user generate them with jbuilder?

[jvillard]

Can't every user generate them with jbuilder?

Just as a data point, this is not always possible at the moment. For instance infer's .merlin is manually curated from the one generated by jbuilder, mostly because of ocaml/dune#230.

[alfa07]

@jvillard we don't use jbuilder internally, we use buck which doesn't generate .merlin files

[let-def]

The ppx-cwd branch address this issue, #870