Pattern guards

[sim642]

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There are severely outdated ports of them to OCaml: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:

• Active or View patterns ocaml/RFCs#12
• Pattern guards ocaml/RFCs#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.