Theo's Typesafe Cult•2y ago

Need help with typescript stuff

I am trying to implement the Maybe monad in typescript. It's like the Option type in Rust. Here's what I have so far:

const None = Symbol("None");

export class Maybe<T> {
  constructor(private value: T | typeof None) {}

  static some<T>(value: T): Maybe<T> {
    if (value === null || value === undefined) return Maybe.none<T>();

    return new Maybe<T>(value);
  }

  static none<T>(): Maybe<T> {
    return new Maybe<T>(None);
  }

  static from<T>(value: T): Maybe<T> {
    if (value === null || value === undefined) return Maybe.none<T>();

    return Maybe.some(value);
  }

  public match<U>({ some, none }: { some: (value: T) => U; none: () => U }): U {
    if (this.value === None) return none();

    return some(this.value);
  }

  public map<U>(fn: (value: T) => U): Maybe<U> {
    if (this.value === None) return Maybe.none<U>();

    return Maybe.some(fn(this.value));
  }

  public flatMap<U>(fn: (value: T) => Maybe<U>): Maybe<U> {
    if (this.value === None) return Maybe.none<U>();

    return fn(this.value);
  }

  public getOrElse<U>(value: U): T | U {
    if (this.value === None) return value;

    return this.value;
  }
}

const None = Symbol("None");

export class Maybe<T> {
  constructor(private value: T | typeof None) {}

  static some<T>(value: T): Maybe<T> {
    if (value === null || value === undefined) return Maybe.none<T>();

    return new Maybe<T>(value);
  }

  static none<T>(): Maybe<T> {
    return new Maybe<T>(None);
  }

  static from<T>(value: T): Maybe<T> {
    if (value === null || value === undefined) return Maybe.none<T>();

    return Maybe.some(value);
  }

  public match<U>({ some, none }: { some: (value: T) => U; none: () => U }): U {
    if (this.value === None) return none();

    return some(this.value);
  }

  public map<U>(fn: (value: T) => U): Maybe<U> {
    if (this.value === None) return Maybe.none<U>();

    return Maybe.some(fn(this.value));
  }

  public flatMap<U>(fn: (value: T) => Maybe<U>): Maybe<U> {
    if (this.value === None) return Maybe.none<U>();

    return fn(this.value);
  }

  public getOrElse<U>(value: U): T | U {
    if (this.value === None) return value;

    return this.value;
  }
}

Here the from method is used to take in a value and convert it to a Maybe<value>. For example:

const a = Maybe.from(9);
//    ^? const a: Maybe<number>

const b = Maybe.from("hi");
//    ^? const b: Maybe<string>

const a = Maybe.from(9);
//    ^? const a: Maybe<number>

const b = Maybe.from("hi");
//    ^? const b: Maybe<string>

But there's a problem, when I pass a value that could either be some type (like number, string, etc) or null or undefined, it returns Maybe<value | undefined>. The Maybe monad handles this by using none so there's no need to have Maybe<value | undefined>, it just needs to be Maybe<value> . For example:

const z = Maybe.from([1, 2, 3].pop()); // z is Maybe<number | undefined> but I want Maybe<number>

const z = Maybe.from([1, 2, 3].pop()); // z is Maybe<number | undefined> but I want Maybe<number>

How do I modify my code in order to implement this behaviour? Playground link: https://www.typescriptlang.org/play?#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-r0JByNZlsjq9QbchCjV4KKaMcTduGRnjgTbMSSeURffbY07ndTafTQu5eFBhF6OQW-eL42LA9LMd7Q4sI+lNdGHXG8gikw8CeiiWHsXt61mJmPuoWY47m6WNm6KzAAObtADy7AAogQIBnyTYQ-EHMHIyC+8Xm0OTaOzXDm1bs3OnP3KpSnnR1ipoDATCoLmnwREQADaACM-IAEz8gAzAAun0bAsKMYwAPQYcCAB6AD8QA

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15 Replies

Unknown User•2y ago

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nexxel•2y ago

not super familiar with Omit lemme look it up

Unknown User•2y ago

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nexxel•2y ago

is this how it would be used?

  static from<T>(value: T): Omit<Maybe<T>, "undefined"> {
    if (value === null || value === undefined) return Maybe.none<T>();

    return Maybe.some(value);
  }

  static from<T>(value: T): Omit<Maybe<T>, "undefined"> {
    if (value === null || value === undefined) return Maybe.none<T>();

    return Maybe.some(value);
  }

nexxel•2y ago

probably using it wrong cause this still can be undefined https://this-vegetable.is-from.space/Alacritty-v0.10.0-portable_(1)_xDQClClw7i.png

Unknown User•2y ago

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nexxel•2y ago

yeah

Unknown User•2y ago

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nexxel•2y ago

i fixed it i think?

  static from<T>(value: T | null | undefined): Maybe<T> {
    if (value === null || value === undefined) return Maybe.none<T>();

    return Maybe.some(value);
  }

  static from<T>(value: T | null | undefined): Maybe<T> {
    if (value === null || value === undefined) return Maybe.none<T>();

    return Maybe.some(value);
  }

just extending the param type to be either T or null or undefined fixed the type errors

Unknown User•2y ago

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nexxel•2y ago

this is interesting lol

nexxel•2y ago

i was trying to implement this https://nexxel-has.no-friends.xyz/brave_QxiJmzp095.png

nexxel•2y ago

but didn't need to do the assert part

Unknown User•2y ago

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nexxel•2y ago

yeah me too

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Need help with typescript stuff