How to force ppx to use an OCaml version?

Question

I have the below OCaml file which compiles correctly without ppx and fails with this dune file

(library
 (name so_proj)
 (preprocess
  (pps
   ppx_inline_test
   ppx_deriving.show
   ppx_deriving.ord
   ppx_deriving.map
   ppx_deriving.eq
   ppx_deriving.fold
   ppx_deriving.iter)))

and works with

(library
 (name so_proj))

the error being

File "SO_naming_existential.ml", line 23, characters 16-18:
23 |  fun (Mod (type xr) (m : xr)) ->
                     ^^
Error: migration error: existentials in pattern-matching is not supported before OCaml 4.13

here's the OCaml file in question which uses a new syntax (and provides an equivalent - I believe - version when it's not available)

type existentiel = Mod : 'x -> existentiel

module type existentiel_m = sig
  type x

  val value : x
end

let to_Module : existentiel -> (module existentiel_m) =
 fun (Mod m) ->
  let namedxr : type xr. xr -> (module existentiel_m) =
   fun v ->
    (module struct
      type x = xr

      let value = v
    end)
  in
  namedxr m

(* Since 4.13 https://github.com/ocaml/ocaml/pull/9584 *)
let to_Module2 : existentiel -> (module existentiel_m) =
 fun (Mod (type xr) (m : xr)) ->
  (module struct
    type x = xr

    let value = m
  end)

To confirm the origin of the error (and to run first for avoiding to waste time..) the command dune build --verbose points indeed at an error happening in ppx

Running[2]: (cd _build/default && .ppx/0789030747a4993265eb655c993f5cab/ppx.exe --cookie 'inline_tests="enabled"' --cookie 'library-name="so_proj"' -o SO_naming_existential.pp.ml --impl SO_naming_existential.ml -corrected-suffix .ppx-corrected -diff-cmd - -dump-ast)
Command [2] exited with code 1:
$ (cd _build/default && .ppx/0789030747a4993265eb655c993f5cab/ppx.exe --cookie 'inline_tests="enabled"' --cookie 'library-name="so_proj"' -o SO_naming_existential.pp.ml --impl SO_naming_existential.ml -corrected-suffix .ppx-corrected -diff-cmd - -dump-ast)
File "SO_naming_existential.ml", line 23, characters 16-18:
23 |  fun (Mod (type xr) (m : xr)) ->
                     ^^
Error: migration error: existentials in pattern-matching is not supported before OCaml 4.13

Can one force ppx to use 4.13, or get a warning when a ppx is not compatible with a given version ? (or is it a bug ?)

ivg · Answer 1 · 2022-02-01T13:27:56.417

An executable or a library could only be composed of compilation units that are compiled by the same compiler. In other words, you can't build some parts of your project with one compiler and the other parts with another.

When you compile an OCaml project using dune the compiler is searched in the in the directories specified in your PATH variable (in Linux). You can see which compiler is selected using the shell command which ocaml. And ocaml -version will tell you its version.

If you're using opam (most likely you are), then you can install the required version of the compiler using the following shell command,

opam switch create 4.13.1

once it is finished, activate the created switch with

eval $(opam env)

This will ensure that the newly installed version of OCaml is available in your path. Double-check that using which ocaml and ocaml -version.

Finally, install the dependencies required by your project with opam install and rebuild the project.

How to force ppx to use an OCaml version?

1 Answers1