jbuilder support

[vogler]

I wanted to use ppx_import with jbuilder.
I tried something like this PR which makes ppx_deriving work with jbuilder. However, I have no idea about packaging and what I did doesn't seem to suffice.

[vogler]

@diml @let-def Maybe you could help? I just mixed things from your ppx_deriving PR and the omp examples.

[ghost]

Someone else asked about this (ocaml/dune#193). It's currently not possible to use ppx_import with jbuilder.

The reason for that is that jbuilder splits the pre-processing from the rest of the compilation for performance reason. As a result ppx rewriters might be run before the cmi files are generated.

It would be possible to change that, but it's an invasive change in jbuilder and one would have to be careful not to slow-down the build.

What could be done relatively easily, that would work well with jbuilder is a version of ppx_import that would work as a standalone code generator rather than a ppx rewriter. With such a tool you could import type definitions coming from other libraries with jbuilder.

[vogler]

a version of ppx_import that would work as a standalone code generator rather than a ppx rewriter

Using this then instead? I'll give it a try. Thanks for your help!

[ghost]

I was more thinking of a plain rule.

But actually the current ppx_import should already work with jbuilder to import types from other libraries, by declaring the dependencies on the cmi files the ppx will read. Essentially you'd write:

(library
 (...
  (preprocessor_deps (${lib:foo:bar.cmi}))
  ...))

${lib:...} is not accepted at present in (preprocessor_deps ...) but it should be eventually

[whitequark]

@diml Can you elaborate on what's needed from me in this case? And what's from jbuilder? Just for triaging.

[ghost]

I suppose the best would be a way to pass individual cmi on the command line rather than include paths, so that jbuilder knows what files the rewriter will read.

[ghost]

But maybe it's not worth changing anything right now. I need to change something in jbuilder which might make it easier to support ppx rewriters that use the typer. Maybe all we'll need is a variable in the META of ppx_import to say that it uses the typer.