amn1729 / maybe-ts

Maybe Monad implemented in typescript

About

Maybe monad for TS Inspired from Haskell’s Maybe and Rust’s Option type

Upstream URL

https://codeberg.org/anhsirk0/maybe-ts

Maybe<T>

• data [private] : Just<T> | Nothing
• isJust [getter] : () => boolean
• isNothing [getter] : () => boolean
• run [method] : <U>(fn: (val: T) => U) => Maybe<NonNullable<U>>
• prop [method] : <U extends NonNullable<T[K]>, K extends keyof T>(key: K) => Maybe<U>
• get [method] : (msg?: string) => T
• getOr [method] : (fallback: T) => T
• getOrUndef [method] : () => T | undefined
• or [method] : (other?: Maybe<T>) => Maybe<T>

Types and functions

Just<T>

• value: T
• _t: “just”

Nothing

• _t: “nothing”

maybe<T>(val?: T | undefined | null) => Maybe<T>

Examples

type User = { name: string; age: number; };
let ben: Maybe<User> = maybe<User>({ name: "Ben" age: 10 });
let benWithDoubleAge: Maybe<User> = ben.run(b => { ...b, age: b.age * 2 });
let finalAge: Maybe<number> = benWithDoubleAge.run(b => b.age);
let finalAgeValue: number = finalAge.getOr(0); //=> 20 or 0;  

type User = { name: string; age: number; };
let ben: Maybe<User> = maybe<User>({ name: "Ben" age: 10 });
let benWithDoubleAge: Maybe<User> = ben.run(b => { ...b, age: b.age * 2 });
let finalAge: Maybe<number> = benWithDoubleAge.prop("age");
let finalAgeValue: number = finalAge.getOr(0); //=> 20 or 0;  

let num: Maybe<number> = maybe<number>(); // Maybe of variant Nothing;
let num2: Maybe<number> = maybe<number>(1); // Maybe of variant Just(1);
let arrayOfNums: Array<number> = [1, 2, 3];
let numAtIndex4: Maybe<number> = maybe(arrayOfNums.at(4));

handling optional props

type User = { name: string; age: number; address?: string };
let ben: Maybe<User> = maybe<User>({ name: "Ben" age: 10 });
let address: Maybe<string> = ben.prop("address");
// let addressValue: string = address.get(); //=> throws Error("No value to unwrap");
let addressValue: string = address.getOr("Not found"); //=> "Not found"  

getting deeply nested props

type Email = { usermail: string; domain: string };
type Info = { phone?: number; email?: Email }
type User = { name: string; info?: Info };

let peter: Maybe<User> = maybe<User>({ name: "Peter Griffin" }); // No info
let emailDomain: Maybe<string> = peter.prop("info").prop("email").prop("domain");
emailDomain.isNothing() //=> true;

let email: Email = { usermail: "stew", domain: "griffins.com" };
let stewie: Maybe<User> = maybe<User>({ name: "Stewie Griffin", info: { email } });
emailDomain: Maybe<string> = stewie.prop("info").prop("email").prop("domain");
emailDomain.isJust //=> true;
emailDomain.get() //=> "griffins.com"