oakfang / adt

Algebraic Data Types for typescript

This is a somewhat naive implementation of ADTs for typescript.

option - the "maybe" type module

Main Exports

• none(): None create an instance of Option<T> that holds no value (unit type)
• some<T>(value: T): Some<T> create an instance of Option<T> that holds a value
• type Option<T> = None | Option<T>

Utilities

• isNone(value) - is provided argument of type None
• getOrElse<T>(option: Option<T>, default: T): T - get an optional value if exists, fallback to default otherwise
• fromNullable<T>(value: T | null | undefined): Option<T> create an Option<T> from a possibly nullable value, where nullish become None and other values become Some<T>

Example Usage

Let's say we have a sparse array of numbers. We'd like to sum it, but some members of the array might be nullish. We can't use falsy values as indication (because 0), so let's try to use options instead.

import { option } from "@oakfang/adt";

const numbers: Array<null | number> = [
  /*...*/
];
const sum = numbers
  // Array<null | number> -> Array<Option<number>>
  .map(fromNullable)
  // Array<Option<number>> -> number[]
  .map((o) => getOrElse(o, 0))
  // number[] -> number
  .reduce((x, y) => x + y, 0);

Result<Left, Right> - the "computation that might fail" type

Some function might throw, but the issue with that is that neither JS nor TS provide us with a way to account for these thrown errors in a type-safe manner. The Result type aims to remove the need for thrown exception, as it encapsulates both a function that follows the "happy flow" (i.e., returns) and one that doesn't.