Result: success/failure type union

example.ts

interface IFoo {
  bar : string;
}

function validateSomething(obj : IFoo) : Result<string, IFoo> {
  if (obj.bar.length === 0) {
    return resultFailure("You must provide at least one object");
  } else {
    return resultSuccess(obj);
  }
}

result.ts

// This is a poor man's "Either", without any monadic capabilities.
export interface Success<TValue> {
    success: true;
    value: TValue;
}

export interface Failure<TFailure> {
    success: false;
    failure: TFailure;
}

export type Result<TFailure, TValue> = Failure<TFailure> | Success<TValue>;

// Below are some convenience functions to reduce boilerplate, or make it more DSL-y. (Otherwise you might need to cast "true" to "true", instead of letting it be typed to "boolean")
export function resultSuccess<T>(value: T): Success<T> {
    return {
        success: true,
        value
    };
}

export function resultFailure<T>(failure: T): Failure<T> {
    return {
        success: false,
        failure
    };
}