Pattern guards

Haskell has another feature called "pattern guards":

• https://wiki.haskell.org/Pattern_guard
• https://web.archive.org/web/20170109011924/http://research.microsoft.com/en-us/um/people/simonpj/Haskell/guards.html
• https://www.microsoft.com/en-us/research/publication/pattern-guards-and-transformational-patterns/
• https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-460003.13
• https://gitlab.haskell.org/haskell/prime/-/wikis/PatternGuards

There are severely outdated ports of them to OCaml:

• For OCaml 3: https://code.google.com/archive/p/ocaml-patterns/wikis/PatternGuards.wiki and https://github.com/jaked/patterns
• For OCaml 4.02.1: https://opam.ocaml.org/packages/ppx_pattern_guard/

There are also OCaml RFCs:

• https://github.com/ocaml/RFCs/pull/12
• https://github.com/ocaml/RFCs/pull/35

Although this ppx is currently explicitly for view patterns, it might be relatively easy to swallow ppx_pattern_guard functionality for transforming matches while sharing some of the underlying infrastructure. Neither feature subsumes the other, but there are big similarities, so it might be worth generalizing this ppx.

Start by reading the repository's existing view-pattern PPX implementation, then compare it with ppx_pattern_guard and OCaml RFCs 12 and 35. Before coding, confirm the intended generalization and define which pattern-guard transformations should be supported; the issue is done when that scope is implemented and validated.