This repository contrains some experiments for a ppx rewriter providing pattern matching on abstract types.

The main motivation for this is to eventually abstract the OCaml AST entirely in ppx_ast, so that ppx rewriters can use a proper API to deal with the OCaml AST and not have to worry about the evolution of the OCaml AST.

The main problem when making the AST abstract is how to deconstruct it. ppx_core provides an Ast_pattern module allowing to do this with combinators. However using them is really painful, and the code is a lot worse that what you can do by just pattern matching on the concrete data types.

This ppx rewriter propose a simple solution to this problem. It rewrites patterm matching of the form:

match%vpat <expr> with
| A x -> f x
| B x -> g x

into some expression where constructors such as A or B are mapped to functions by simply lowercasing their name.

You can see a full example in test/test.ml.

The plan is:

• finish this rewriter, in particular add support for record patterns
• update ppx_ast to abstract the OCaml AST entirely and provides all the necessary builders/patterns
• upgrade all Jane Street ppx rewriters to use this

This will be a breaking changes for users of ppx_core and [ppx_driver][ppx_driver]. However, once users will have switched, they'll get much better stability guarantees since changes in the parsetree will simply be reflected by forward compatible changes of the ppx_ast API.

Under the hood, ppx_ast will use one the version of the Parsetree supported by ocaml-migrate-parsetree. It will always try to use the highest available version, so that the latest compiler features are available immediately.

Currently nothing appart from this experiment as been done.