Heapcon 2022: Railway Oriented Typescript

railway_oriented_typescript.ts

/*
  =============================
   Railway Oriented Typescript
  =============================
               by @robinpokorny
*/

/* === 1. Union basics === */

const a: string | number = 42;

const b: "hey" | "ahoy" = "hey";
const c: "hey" | "ahoy" = "heyyyy"; // Error
const code: 200 | 400 | 500 = 200;

const f = (a: { name: string } | { id: number }) => {
  a.name; // Property 'name' does not exist on type '{ name: string; } | { age: number; }'.

  if ("name" in a) {
    a.name;
  }
};

type Person = { tag: "person"; name: string };
type Robot = { tag: "robot"; id: number };

const g = (a: Person | Robot) => {
  if (a.tag === "person") {
    a.name;
    a.id; // Error
  }
};

/* === 2. Success, Failure, Result === */

type Success<T> = {
  tag: "success";
  value: T;
};

type Failure<E> = {
  tag: "failure";
  value: E;
};

type Result<T, E> = Success<T> | Failure<E>;

/* === 3. Use Result === */

const validate = (input: unknown): Result<number, "NOT_AN_INTEGER"> => {
  if (Number.isInteger(input)) {
    return {
      tag: "success",
      value: input as number,
    };
  } else {
    return {
      tag: "failure",
      value: "NOT_AN_INTEGER",
    };
  }
};

const updateDB = (input: number): Result<Date, "DB_UNAVAILABLE"> => {
  DB.write(input);

  if (DB.success) {
    return {
      tag: "success",
      value: new Date(),
    };
  } else {
    return {
      tag: "failure",
      value: "DB_UNAVAILABLE",
    };
  }
};

const log = (value: unknown) => console.log("[RP]:", value);

/* === 4. Chain and Tee === */

const chain =
  <T, S, E>(f: (a: T) => Result<S, E>) =>
  <F>(result: Result<T, F>): Result<S, E | F> => {
    if (result.tag === "failure") {
      return result;
    }

    return f(result.value);
  };

const chainedUpdateDB = chain(updateDB);

const tee =
  <T, E>(f: (a: unknown) => void) =>
  (result: Result<T, E>): Result<T, E> => {
    if (result.tag === "success") {
      f(result.value);
    }
    return result;
  };

// (result: Result<unknown, unknown>) => Result<unknown, unknown>
const teedLog = tee(log);

const action = (input: unknown) => {
  // pipe(a, b, c) == c(b(a))

  // Result<Date, "DB_UNAVAILABLE" | "NOT_AN_INTEGER">
  const response = pipe(input, validate, chain(updateDB), tee(log));

  if (response.tag === "failure") {
    switch (response.value) {
      case "DB_UNAVAILABLE":
        return "…";
      case "NOT_AN_INTEGER":
        return "…";
      case "NOT_AN_ARRAY":
        return "…";
    }

    if (response) {
      // Unreachable code detected
    }
  }

  return response.value;
};

// A: `string`-`number` mismatch
// B: Forgot `chain`

// UTILS
const DB = {
  success: false,
  write: (x: unknown) => {},
};
import { pipe } from "fp-ts/function";
export const input = 3;
[a, b, c, code, action, f, g, chainedUpdateDB, teedLog];