[ANN] Vaast 0.0.0 + Design discussion

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Hi all,

This is a bit of mixed post.

I am excited to announce the 1st prototype of Vaast !
Vaast is a library to handle multiple versions of the Typedtree at once. It is meant as a replacement for the current compiler-version-dependent techniques (be it preprocessing or branching).

Similarly to ppxlib for the Parsetree, it provides a single representation of the Typedtree. Unlike ppxlib, it does not select a compiler version’s representation but provides a form of super-representation which embeds the Typedtree from OCaml 4.14 to 5.5 at once. It does so via a dedicated type encoding which explicitly and precisely identifies the differences in-between versions.Vaast’s Typedtree representation is a shallow replacement for OCaml’s Typedtree, making it cheap to use and easy to integrate incrementally in existing code.

More details on its design are available on my blog:

This leads us to the second part of this post: discussing the design.
The prototype is usable as a replacement for the compiler’s Typedtree (and I experimented it on the dead_code_analyzer). Overall its design resembles the Typedtree but limits the breakages in user-code when updating to a more recent OCaml version.

Here is an example of code in odoc, relying on cppo to handle the different versions of the Typedtree:

#if defined OXCAML
          | Tpat_alias (_, id, loc, _uid, _, _, _) -> (
#elif OCAML_VERSION >= (5, 4, 0)
          | Tpat_alias (_, id, loc, _uid, _ty) -> (
#elif OCAML_VERSION >= (5, 2, 0)
          | Tpat_alias (_, id, loc, _uid) -> (
#else
          | Tpat_alias (_, id, loc) -> (
#endif
              match maybe_localvalue id loc.loc with
              | Some x -> poses := x :: !poses
              | None -> ())

And here is the equivalent using Vaast (and discarding the OxCaml case):

          | Tpat_alias {id; name; _} -> (
              match maybe_localvalue id name.loc with
              | Some x -> poses := x :: !poses
              | None -> ())

There are some rough edges (Texp_function, flattenable versioned types), and, before diving deeper into building the library, I would appreciate feedback and participation of those interested in its development and its use.
I opened a dedicated issue for that purpose.

The mid-term goal is to add utility functions for usual operations, support a few mainstream modules of compiler-libs (Parsetree, Types), and make Vaast a basic building block for static analyzers.
The long-term goal is to interface most of compiler-libs.

Thanks !

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