example ppx external issue

gistfile1.txt

(*
module <Foo> = {
  let makeProps = <Foo>.makeProps;
  let make =
    React.createElement(
      {
        module Comp = (val ReasonResource.readOrThrow([%reasonResource <Foo>]));
        Comp.make;
      },
      _,
    );
};
*)
let module_expr =
  (fun mapper module_expr -> match module_expr with
    | ({
        pmod_desc = Pmod_extension ({txt = "lazyLoadComponent"}, PStr [ {pstr_desc = Pstr_eval ( {
          pexp_desc = Pexp_construct ({txt}, _)
          }, _)} ])
      } as pmod) -> begin
        { pmod with pmod_desc = Pmod_structure [
          Str.value Nonrecursive [
            Vb.mk (Pat.var {loc=(!default_loc); txt = "reasonResource"})
            (expr mapper (
              Exp.extension (
                {loc=(!default_loc); txt = "reasonResource"},
                PStr [Str.eval (Exp.construct ({loc=(!default_loc); txt}) None)]
            )))
          ];
          Str.value Nonrecursive [
            Vb.mk (Pat.var {loc=(!default_loc); txt = "makeProps"})
            (Exp.ident {loc=(!default_loc); txt = Ldot (txt, "makeProps")})
          ];
          Str.value Nonrecursive [
            Vb.mk (Pat.var {loc=(!default_loc); txt = "make"})
            (Exp.fun_ "" None (Pat.var {loc=(!default_loc); txt = "props"}) (Exp.apply
              (Exp.ident {loc=(!default_loc); txt = Ldot ((Lident "React"), "createElement")}) [
                ("", (Exp.letmodule
                  {loc=(!default_loc); txt = "Comp"}
                  (Mod.unpack (Exp.apply
                    (Exp.ident {loc=(!default_loc); txt = Ldot ((Lident "ReasonResource"), "readOrThrow")})
                    [("", Exp.ident {loc=(!default_loc); txt = Lident "reasonResource"})]
                  ))
                  (Exp.ident {loc=(!default_loc); txt = Ldot ((Lident "Comp"), "make")})
                ));
                ("", Exp.ident {loc=(!default_loc); txt = Lident "props"})
              ]
            ))
          ]
        ]}
    end
    | _ -> default_mapper.module_expr mapper module_expr
  )

On line 3 - if Foo.makeProps is a value instead of an external the code size in this file will be larger and the asynchronous code loading will be unsuccessful. We'd like to be able to copy this external over and not allow compilation if it's not an external. We can't copy it over because we don't know the type in the ppx.

We can mitigate this issue by calling ReasonResource.readOrThrow in the top level of this module, but then we could throw in a way that React JS isn't ready to handle (outside of it's control flow) - we would also be loading the code before it's strictly needed.