nodejscont.js

require("@babel/polyfill");
const {
  Fnctr,
  Arr,
  implement,
  Functor,
  Chain,
  Apply
} = require("@masaeedu/fp");
const Cont = require("@masaeedu/fp/dist/instances/cont");
const fs = require("fs");

const FalsyFirstEither = (() => {
  const Left = e => [e];
  const Right = (...x) => [null, ...x];
  const match = ({ Left, Right }) => ([e, ...x]) => (e ? Left(e) : Right(...x));

  return { Left, Right, match };
})();

const GenericEitherT = ({ Left, Right, match }) => M => {
  const of = x => M.of(Right(x));
  const chain = f =>
    match({
      Left: l => M.of(Left(l)),
      Right: r => f(r)
    }) |> M.chain;

  return (
    { of, chain }
    |> implement(Chain)
    |> implement(Apply)
    |> implement(Functor)
    |> implement(Apply) // lift2 depends on functor, so we have to do this twice. TODO: do dependency solving and get rid of this shit
  );
};

const trim = s => s.trim();

{
  const { of, map, chain } = GenericEitherT(FalsyFirstEither)(Cont);

  const spreadify = f => (...args) => f(args);
  const run = rawNodeAPI => (...args) => cb =>
    rawNodeAPI(...args, spreadify(cb));

  const fork = x =>
    x(([err, ...data]) => {
      if (err) throw err;
      console.log(...data);
    });

  run(fs.writeFile)("foo.txt", "bar.txt")
    |> chain(_ => run(fs.writeFile)("bar.txt", "This is a test."))
    |> chain(_ => run(fs.readFile)("foo.txt", "utf8"))
    |> map(trim)
    |> chain(run(fs.readFile))
    |> map(x => x.length)
    |> fork; // => 15
}