Can't seams to alight the types when folding the port. My expectation is for port to be number, which it is, but I'm getting an error telling me it can't be assigned to Error.

example.ts

import type { PathLike } from "fs";
import fs from "fs";
import path, { parse } from "path";
import * as E from "fp-ts/lib/Either";
import { pipe } from "fp-ts/function";
import { object } from "firebase-functions/v1/storage";
import { objectHasProp } from "~/lib/utils";
import * as IE from "fp-ts/IOEither";

main();
function main() {
  pipe(
    getPort(new URL(configPath)),
    E.fold(
      (err) => err,
      (port) => port // HERE: number is not assignable to type Error
    ),
  );
}
function getPort(p: PathLike) {
  return pipe(
    readFileSync(p, "utf-8"),
    E.flatMap(parseJSON),
    E.flatMap(parseConfig),
  );
}

function parseConfig(config: unknown): E.Either<Error, number> {
  if (objectHasProp(config, "port")) {
    return E.right(config.port as number);
  } else {
    return E.left(new Error("No Port"));
  }
}

function readFileSync(path: PathLike, encoding: BufferEncoding) {
  return tryCatch(() => fs.readFileSync(path, encoding));
}

function parseJSON(str: string): E.Either<Error, unknown> {
  return tryCatch(() => JSON.parse(str));
}

function tryCatch<T>(f: () => T): E.Either<Error, T> {
  try {
    return E.right(f());
  } catch (err) {
    return E.left(err instanceof Error ? err : new Error(String(err)));
  }
}
export const objectHasProp = <T extends string>(
  v: unknown,
  hasProp: T,
): v is { [K in T]: unknown } => {
  return typeof v === "object" && v !== null && hasProp in v;
};