Re: [Caml-list] [ANN] Jane Street's ppx rewriters

[Jeremie Dimino <jdimino@janestreet.com>]

On Mon, Oct 5, 2015 at 2:48 PM, Gabriel Scherer
<gabriel.scherer@gmail.com> wrote:
> Do you plan provide any backward-compatibility guarantees for
> ppx-core, or do you plan the helpers there to mirror changes to
> upstream parsetree?

Good question. Currently most of the helpers are auto-generated from
the signature of Parsetree. Using names that closely matches the ones
in parsetree.mli. These will surely change as Parsetree changes. Then
we have additional helpers with different names for common use, these
will probably be backward compatible. Going forward we'll add more
such helpers to improve compatibility between versions of the
parsetree.

You can see the interface for these helpers here:

  https://github.com/janestreet/ppx_core/blob/master/src/ast_builder.ml
  https://github.com/janestreet/ppx_core/blob/master/src/ast_builder_intf.ml

And for patterns:

  https://github.com/janestreet/ppx_core/blob/master/src/ast_pattern.ml
  https://github.com/janestreet/ppx_core/blob/master/src/ast_pattern_intf.ml

> (I think there would be a place in the ppx ecosystem for a library
> that allows users to write rewriters on their current OCaml version,
> and know that they will still compile and be useable to preprocess
> OCaml code for future versions. This may require an API with explicit
> versioning.)

Sure. But having backward-compatible helpers for building the AST
is not enough as your patterns will still break. We have [Ast_pattern]
which is often enough for matching the payload of attributes and
extensions, but it doesn't conveniently replace all pattern matchings.

-- 
Jeremie