OK-FP

Essential Effect Data Types for TypeScript

OK-FP is a small, focused functional programming toolkit for TypeScript.

It provides a minimal set of typed effects: composable, type-safe wrappers for optional values, errors, and async computations.

Effects

Effect	Description
Option<T>	A value that may or may not be present. Use instead of null/undefined.
Either<E, T>	A computation that succeeds with T or fails with a typed error E. Errors are explicit and must be handled.
Validation<E, T>	Like Either, but accumulates all errors instead of stopping at the first one. Ideal for form and config validation.
Task<T>	A lazy async computation that always succeeds. Executes only when .run() is called - unlike Promises, which are eager.
TaskEither<E, T>	A lazy async computation that can succeed with T or fail with E. Combines Task's laziness with Either's typed errors.

Installation

Install with your package manager of choice:

npm install ok-fp
# or
# pnpm add ok-fp
# yarn add ok-fp

Basic example

This example shows a small pipeline using Option.

import { type Option, some, none } from "ok-fp/option";

const parseNumber = (input: string): Option<number> => {
  const n = Number(input);
  return Number.isFinite(n) ? some(n) : none();
};

const nonZero = (n: number): Option<number> => (n !== 0 ? some(n) : none());

const positive = (n: number): Option<number> => (n > 0 ? some(n) : none());

const compute = (input: string): Option<number> =>
  parseNumber(input)
    .flatMap(nonZero) // must not be 0
    .flatMap(positive) // must be > 0
    .map((n) => 1 / n) // reciprocal (no need for flatMap)
    .map((n) => n * 100); // scale

// Usage (returns Option instances)
compute("4"); // returns some(25)
compute("0"); // returns none() (fails nonZero)
compute("-3"); // returns none() (fails positive)
compute("abc"); // returns none() (fails parsing)

Documentation

Full documentation and API reference are available in the docs site or the docs/ folder.

API docs

License

This project is licensed under the MIT License. See the LICENSE file for details.