Result type in TypeScript

result.ts

interface Result<T, E> {
    map<U>(fn: (a: T) => U): Result<U, E>;
    mapErr<U>(fn: (a: E) => U): Result<T, U>;
    isOk(): boolean;
    isErr(): boolean;
    ok(): Option<T>;
    err(): Option<E>;
    and<U>(res: Result<U,E>): Result<U,E>;
    andThen<U>(op: (T) => Result<U,E>): Result<U,E>;
    or(res: Result<T,E>): Result<T,E>;
    orElse<U>(op: (T) => Result<T,U>): Result<T,U>;
    unwrap(): T;
    unwrapOr(optb: T): T;
    unwrapOrElse(op: (E) => T): T
}

class Ok<T, E> implements Result<T, E> {
    private value: T;

    constructor(v: T) {
        this.value = v;
    }

    map <U>(fn: (a: T) => U): Result<U, E> {
        return new Ok<U, E>(fn(this.value))
    }

    mapErr <U>(fn: (a: E) => U): Result<T, U> {
        return new Ok<T, U>(this.value);
    }

    isOk(): boolean {
        return true;
    }

    isErr(): boolean {
        return false;
    }

    ok(): Option<T> {
        return new Some(this.value);
    }

    err(): Option<E> {
        return None.instance<E>();
    }

    and<U>(res: Result<U,E>): Result<U,E> {
        return res;
    }

    andThen<U>(op: (T) => Result<U,E>): Result<U,E> {
        return op(this.value);
    }

    or(res: Result<T,E>): Result<T,E> {
        return this;
    }

    orElse<U>(op: (T) => Result<T,U>): Result<T,U> {
        return new Ok<T,U>(this.value);
    }

    unwrapOr(optb: T): T {
        return this.value;
    }

    unwrapOrElse(op: (E) => T): T {
        return this.value;
    }

    unwrap(): T {
        return this.value
    }

    toString(): string {
        return "Some " + this.value;
    }
}

class Err<T, E> implements Result<T, E> {
    private error: E;

    constructor(error: E) {
        this.error = error;
    }

    map <U>(fn: (a: T) => U): Result<T, E> {
        return this;
    }

    mapErr <U>(fn: (a: E) => U): Result<T, U> {
        return new Err<T,U>(fn(this.error));
    }

    isOk(): boolean {
        return false;
    }

    isErr(): boolean {
        return false;
    }

    ok(): Option<T> {
        return None.instance<T>();
    }

    err(): Option<E> {
        return new Some(this.error);
    }

    and<U>(res: Result<U,E>): Result<U,E> {
        return new Err<U,E>(this.error);
    }

    andThen<U>(op: (T) => Result<U,E>): Result<U,E> {
        return new Err<U,E>(this.error);
    }

    or(res: Result<T,E>): Result<T,E> {
        return res;
    }

    orElse<U>(op: (T) => Result<T,U>): Result<T,U> {
        return op(this.error);
    }

    unwrapOr(optb: T): T {
        return optb;
    }

    unwrapOrElse(op: (E) => T): T {
        return op(this.error);
    }

    unwrap(): T {
        throw "Err.get"
    }

    public toString(): string {
        return "None";
    }
}

Some of your functions are typed incorrectly - for instance, in andThen, you have a parameter named T instead of a parameter typed with T. In other words, you have

andThen<U>(op: (x: T) => Result<U,E>): Result<U,E>;

when it should be

andThen<U>(op: (x: T) => Result<U,E>): Result<U,E>;

Try using the --noImplicitAny flag to warn about these issues.

@DanielRosenwasser Why does TypeScript interpret that as the name, though?
In type expression context, wouldn't optional name but mandatory type make a lot more sense?

@s-panferov

interface BaseResult<T, E> {
  isOk(): this is Ok<T, E>
  isErr(): this is Err<T, E>
  ok(): Option<T>
  err(): Option<E>
  map<U>(fn: (val: T) => U): Result<U, E>
  mapErr<U>(fn: (err: E) => U): Result<T, U>
  and<U>(res: Result<U, E>): Result<U, E>
  andThen<U>(op: (val: T) => Result<U, E>): Result<U, E>
  or(res: Result<T, E>): Result<T, E>
  orElse<U>(op: (err: E) => Result<T, U>): Result<T, U>
  unwrap(): T | never
  unwrapOr(optb: T): T
  unwrapOrElse(op: (err: E) => T): T
}

type Result<T, E> = Ok<T, E> | Err<T, E>

class Ok<T, E> implements BaseResult<T, E> {
  constructor(private value: T) {}

  map<U>(fn: (a: T) => U) {
    return new Ok<U, E>(fn(this.value))
  }

  mapErr<U>(fn: (a: E) => U) {
    return (this as unknown) as Ok<T, U>
  }

  isOk(): this is Ok<T, E> {
    return true
  }

  isErr(): this is Err<T, E> {
    return false
  }

  ok(): Option<T> {
    return new Some(this.value)
  }

  err(): Option<E> {
    return None.instance<E>()
  }

  and<U>(res: Result<U, E>) {
    return res
  }

  andThen<U>(op: (val: T) => Result<U, E>) {
    return op(this.value)
  }

  or(res: Result<T, E>) {
    return this
  }

  orElse<U>(op: (err: E) => Result<T, U>) {
    return (this as unknown) as Ok<T, U>
  }

  unwrapOr(optb: T) {
    return this.value
  }

  unwrapOrElse(op: (err: E) => T) {
    return this.value
  }

  unwrap(): T {
    return this.value
  }

  toString() {
    return "Some " + this.value
  }
}

class Err<T, E> implements BaseResult<T, E> {
  constructor(private error: E) {}

  map<U>(fn: (a: T) => U) {
    return (this as unknown) as Err<U, E>
  }

  mapErr<U>(fn: (a: E) => U) {
    return new Err<T, U>(fn(this.error))
  }

  isOk(): this is Ok<T, E> {
    return false
  }

  isErr(): this is Err<T, E> {
    return false
  }

  ok(): Option<T> {
    return None.instance<T>()
  }

  err(): Option<E> {
    return new Some(this.error)
  }

  and<U>(res: Result<U, E>) {
    return (this as unknown) as Err<U, E>
  }

  andThen<U>(op: (val: T) => Result<U, E>) {
    return (this as unknown) as Err<U, E>
  }

  or(res: Result<T, E>) {
    return res
  }

  orElse<U>(op: (err: E) => Result<T, U>) {
    return op(this.error)
  }

  unwrapOr(optb: T) {
    return optb
  }

  unwrapOrElse(op: (err: E) => T) {
    return op(this.error)
  }

  unwrap(): never {
    throw this.error
  }

  toString() {
    return "None"
  }
}