From what I see the error message is correct, as this makes the Ast module invisible to Menhir.

Oh, why is that? I had assumed it would work similarly to other qualified modules - where you could mention sibling modules from within a subdirectory? See the directory tree from #11119:

├── dune
├── dune-project
├── foo.ml
├── lang
│   ├── ast.ml
│   ├── dune
│   └── parser.mly
└── run.t

If this is intentional is there a workaround for this, or is this pattern supposed to be impossible with Menhir?

I’ve also tried the following from the root dune file:

(subdir lang
 (menhir
  (modules parser)))

Which resulted in the same error.

I also tried:

(menhir
 (modules lang/parser))

Which resulted in:

Error: dune__exe__Lang/parser corresponds to an invalid module name
-> required by _build/default/.foo.eobjs/dune__exe.ml-gen
-> required by _build/default/.foo.eobjs/byte/dune__exe.cmi
-> required by _build/default/lang/parser__mock.mli.inferred
-> required by _build/default/lang/parser.ml
-> required by alias lang/all
-> required by alias default

Would you prefer it if I used an example that used one of the approaches above, or are those also intended behavior?

I think the main reason I used a dune file in the subdirectory was because I had no other way of referencing the .mly from the root directory via the menhir stanza. And I assume using the subdir stanza is isomorphic to the approach I am currently using (but I could be mistaken on that).

I’ve also tried variations of (modules Lang.Parser), (modules Lang__Parser), (modules dune__exe__Lang__parser) to no avail in the menhir stanza.

@Leonidas-from-XIV If it helps, it seems #8987 was merged, which seems like a case where the .mly is the “group interface” of the subdirectory, which references a sibling module:

├── dune
├── dune-project
└── group
    ├── dune         (menhir stanza is here)
    ├── group.mly    (refers to M)
    └── m.ml

https://github.com/ocaml/dune/pull/8987/files

In contrast the pattern I am after is:

├── dune
├── dune-project
└── group
    ├── dune         (menhir stanza is here)
    ├── m.ml
    └── parser.ml    (refers to M)

Not sure if that test is enough to covers this use-case case as well, as in this case the parser.mly is not the group interface? But it’d be really cool if that issue was fixed, this pattern would be supported as well!