example for discussion https://dev.to/yawaramin/how-to-handle-a-nodeback-in-reasonml-in7

nodeCallback.re

module Encoding = {
  type norm =
    | HEX
    | UTF8
    | ASCII
    | LATIN1
    | BASE64;
  let getNorm = (e: norm) =>
    switch (e) {
    | UTF8 => "utf8"
    | HEX => "hex"
    | ASCII => "ascii"
    | LATIN1 => "latin1"
    | BASE64 => "base64"
    };
};

let dirname: option(string) = [%bs.node __dirname];

[@bs.module "path"] external resolve: (string, string) => 'path = "resolve";

[@bs.module "fs"]
external fsReadFile: ('test, string, 'callback) => 'nodeCallback = "readFile";

let readFile = (path: string, norm: Encoding.norm, callback) =>
  fsReadFile(path, Encoding.getNorm(norm), callback);
// let dirname = Some("./src/test.txt");

switch (dirname) {
| None => print_endline("no __dirname")
| Some(path) =>
  readFile(resolve(path, "test.txt"), UTF8, (err, result) =>
    err ? print_endline("Error") : print_endline(result)
  )
};

test.txt

This is text from text.txt.