Unable to use future_syntax with dune 1.10.0

[Dean177]

I am attempting to use the new ocaml-syntax-shims with other preprocessors on dune 1.10.0

but get the following error:

 (pps ppx_inline_test -- -pp %{bin:ocaml-syntax-shims})
                                         ^^^^^^^^^^^^^^^^^^^^^^^
Error: %{bin:..} isn't allowed in this position

Reproducing example:

(library
  (name future_example)  
  (inline_tests)
  (preprocess (pps ppx_inline_test -- -pp %{bin:ocaml-syntax-shims})))

let (let+) l f = 
  List.map f l

let add_one l = 
  let+ elem = l in
  elem + 1

let%test _ =
  add_one [1;2;3] = [2;3;4]

See also #1934

[aalekseyev]

Apparently we don't expand %{bin} here, probably for no particular reason.

As a workaround you could try something like this:

(library
 (name future_example)
 (preprocess (pps ppx_inline_test -- -pp ./ocaml-syntax-shims))
 (preprocessor_deps ./ocaml-syntax-shims)
 )
(rule (copy %{bin:ocaml-syntax-shims} ./ocaml-syntax-shims))

@mlasson, I'm told you know something about ppx flag expansion.
Do you think it would make sense to support more variables in ppx flag expansion?

[Dean177]

Thanks @aalekseyev, that works perfectly

[mlasson]

@aalekseyev : Yes it would totally makes sense, but I think some thinking should be put into that.

Currently, most macro are allowed only in actions (except for %{env} which feels like a hack).
Moreover, these expansions in flags are "static": they are triggered even if you don't build a thing and %{bin} could fail (because the binary is not in the installed world).

If we wanted to support more macros in preprocessors flags we would need to change the logic in order to delay the expansion at the build stage. Moreover, for consistency, we would probably want to do that for other kinds of flags. So this does not seem trivial to me (but I'm far from being an expert).

Finally, if we want to go in that direction (and why not ?), we may want to support other macros that may add dependencies to the build (so we will need not to forget to collect them and record them).

[emillon]

I'm not sure about the timeline here, but this has been fixed in #2076 (dune 1.10). Using (lang dune 1.10) enables this.