This appears to be a bug triggered by a combination of

ocamlc
ppxlib
ppx_expect

And, I guess, ocamlfind

You'll need to install patdiff, dune, ocamlfind and ppx_expect.

opam install ocamlfind ppx_expect dune

1. Run dune runtest. Observe that there's an (expected) test failure.

2. Run ./MAKE.OK. Observe the same (expected) test failure.

3. Run ./MAKE.BUSTED. Observe that it fails to even build.

1. dune builds and runs this expect-test correctly

2. a hand-built script that invokes the PPX rewriters does the same. This script was built by reverse-engineering from dune's _build/log.

3. but a naive ocamlfind ocamlc invocation fails, complaining

File "<command-line>", line 1, characters 0-3:
	Error: constant expected

dune doesn't use ocaml's -ppx facility -- it builds and invokes PPX rewriters directly before passing the results to ocamlc.

Well, ppx_expect needs a "library-name" arugment, and the format of that argument (after all shell-quoting has been removed) is

library-name="foo"

This needs to be quoted in a list of shell-arguments, or the double-quotes will be parsed by the shell. But it has to be quoted again, to be passed to ocaml's -ppx argument. Ocamlfind doesn't do any of this, assuming that the user will do so.

One could blame this on ocamlfind, or on ocamlc, but I think the blame falls on ppxlib (which is the library asking for this argument). It uses the ocaml AST (parsetree.mli) parser to parse these arguments, and if it's going to do that, it should provide a way to pass such arguments in a file, and that way should be sufficiently obvious that it's the default. But this is pretty strenuous and would involve serious changes to ocamlfind and makefiles upstream. So the best way to solve this problem, is to ALWAYS ensure that no quotes are used in arguments to PPX and PP rewriters.

Otherwise, bit-by-bit, you're going to find that you can't use PPX rewriters outside of dune, b/c ocamlfind will fail to invoke them correctly.

Again, I don't blame ocamlfind: it wasn't expecting an argument in double-quotes, that would thus need to be single-quoted twice. Typically, humans are not good at dealing with that level of quotation.