Interpolation support for any expression (not only identifiers)

[davesnx]

Explore how difficult is to support any expression inside the interpolation syntax.

[%css "font-size: $(string_of_int(23))"];

let base = 33;
[%css "font-size: $(string_of_int(base))"];

let darken = (color, num) => { ... };
[%css "color: $(darken(CssJs.blue, 0.3))"];

Any expression it might be virtually impossible to parse by styled-ppx, but we can “stop parsing” until a matching ) appears.

Another approach is trying to run the "current parser" as a single expression inside the interpolated value, I'm unsure how to do this since embeding ReScript/OCaml/Reason parsers is not an option (would bloat the binary)

Other ppxes that support some sort of expression interp is because the entire payload is valid: https://github.com/thierry-martinez/metapp.

metaquote uses a trick where [%expr] is the ppx, and [%e ] can be placed inside expr for any expression. Errors are tricky, and DX seems bad. Similar case is https://github.com/stedolan/ppx_stage

The last idea is to have a mini parser for only function calls in OCaml/Reason and ReScript that we support, similar on how ppx_str works https://github.com/AriaFallah/ppx_str/blob/master/src/ppx/ppx_str.ml and warn the user about it, so they can always extract into a function.

[davesnx]

https://ocaml-ppx.github.io/ppxlib/ppxlib/Astlib/Parse/index.html#val-expression