optres

This package provides utilities for working with the Option and Result monads in Typescript, inspired by languages like Rust and functional programming paradigms. These abstractions help manage optional values and error handling in a clean, type-safe way.

Option Monad

The Option monad encapsulates an optional value. A value of type Option<T> either contains a value (some(value)) or it is empty (none). This is useful for functions that may not always return a value.

• some(value): Indicates a present value.
• none(): Indicates the absence of a value.

Use Cases

• Return a value if available, otherwise return nothing, without using null or undefined.
• Chain operations that may or may not produce a value.

Result Monad

The Result monad represents the outcome of an operation that can succeed or fail. A value of type Result<T, E> is either ok(value) (success) or err(error) (failure).

• ok(value): Represents a successful outcome.
• err(error): Represents a failure, containing error information.

In the same way that you can throw many different types in Javascript, a Result<T, E> can have any type as the possible T and E values. You can instantiate Error objects, or you might prefer to pass strings instead for more lightweight code.

Use Cases

• Handling errors explicitly without exceptions, creating logic that is easier to reason about.
• Composing operations that may fail and propagating errors.

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Code Examples

Option

import { none, Option, some } from "optres";

// Creating an Option with a value
const value: Option<number> = some(42);

if (value.isSome()) {
  console.log("Got a value:", value.unwrap()); // prints 42
}

// Creating an Option with no value
const nothing: Option<number> = none();

if (nothing.isNone()) {
  console.log("No value present");
}

// Using Option in a function
function findEven(numbers: number[]): Option<number> {
  for (const n of numbers) {
    if (n % 2 === 0) return some(n);
  }
  return none();
}

Result

import { err, ok, Result } from "optres";

// Creating a successful Result
const success: Result<string, string> = ok("Everything went fine!");

if (success.isOk()) {
  console.log(success.unwrap()); // prints "Everything went fine!"
}

// Creating an error Result
const failure: Result<string, string> = err("Something went wrong");

if (failure.isErr()) {
  console.error(failure.unwrapErr()); // prints "Something went wrong"
}

// Using Result in a function
function parseNumber(input: string): Result<number, string> {
  const parsed = Number(input);
  if (isNaN(parsed)) {
    return err("Input is not a valid number");
  }
  return ok(parsed);
}

Advanced use

Note that, while you can use functions like isSome, isOk and unwrap as in the examples above, it is more idiomatic to use functional patterns and the principle of "tell, don't ask".

function getAdmin(
  rawUsername: string,
): Result<User, "invalid username" | "not found" | "not admin"> {
  return validateUsername(rawUsername)
    .chain((username) =>
      getUserFromDatabase(username).okOr("not found" as const)
    )
    .chain((user) => user.isAdmin() ? ok(user) : err("not admin"));
}
const panel = getAdmin("sck").match({
  Ok: (admin) => getAdminPanel(admin),
  Err: (errorType) => getUserPanel(errorType),
});

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Summary

• Use Option for values that may or may not exist.
• Use Result for operations that may fail, and handle errors explicitly.
• The functions some, none, ok, and err are used to construct these monads.