package ppxlib

Non-continuous trunk-support for Ppxlib

On this branch, we provide a Ppxlib version, which can be compiled with OCaml's trunk branch. Whenever you want to compile a project that (transitively) depends on PPXs with trunk, please pin Ppxlib to this branch.

The branch is maintained manually. So each time the compiler adds a new syntax feature, first we need to bring this branch back in sync with trunk and then you'll need to re-pin your Ppxlib. Please, open an issue or a PR, if this branch doesn't compile with trunk. Also open an issue, if there's a feature/bug-fix on Ppxlib's main branch that you'd like to have here, so that we rebase.

trunk-support vs trunk-feature support

Ppxlib supporting trunk can be interpreted in two different ways.

User perspective on the difference

One notion of trunk-support is that a project depending on Ppxlib can be compiled with trunk under the assumption that the project doesn't use the new syntax features in trunk. That's what this branch provides.

The other notion of trunk-support should actually be called trunk-feature support: you can use the new trunk syntax features in a project which depends on Ppxlib and that you compile with trunk. It's not unlikely that you'll get a Ppxlib runtime error (i.e. project compile time error), if you try to use this branch in that situation.

Ppxlib behavior perspective on the difference

The Ppxlib behavior in the situation of trunk support is: The parsetree version exposed by Ppxlib is still the one from the latest stable compiler and all PPXs are defined against that one. However, Ppxlib has hold of trunk's parsetree version and knows how to migrate between that parsetree version and the parsetree version of the latest stable compiler. You can find those parsetrees and parsertree migration modules in Ppxlib's Astlib library. So, when compiling a file with trunk, the Ppxlib driver will parse the file into the trunk parsetree, migrate down to the latest stable parsetree, do the PPX expansions and migrate back to trunk's parsetree. The downward migration will error, if the parsetree contains a feature that isn't well-defined in the latest stable parsetree version.

The behavior in the case of trunk-feature support would be different. In that case, Ppxlib would have to expose the trunk parsetree, so that the PPXs would be defined against that one. So, Ppxlib would have to bump its internal parsetree to the trunk parsetree. Bumping the Ppxlib parsetree always means potentially breaking PPXs. When bumping the Ppxlib parsetree to the parsetree of an about-to-be-released stable compiler version, we create a work-space with all opam PPXs and send patch PRs to the ones we break. However, for trunk we wouldn't do such an effort.