2. Either

There are situations where a Maybe is not the most apropriate Algebraic Data Type to handle certain situations, you will have (in occassions) more than simply a value or nothing, you will have value or a 'value, to illustrate this we'll create a sum function.

/**
 * Tipical Vanilla Approach
 */

let sum = (a, b) => a + b;

sum(1, 3); // 4. Great!
sum(null, 5); // 5. What?
sum(10, undefined); // NaN. Well, this is actually true.

Well, this situations can be easily addressed with a couple of If's

let isFalsy = (v) =>
  (v instanceof Number && isNaN(v)) || typeof v === 'undefined' || v === null;

let sum = (a, b) => {
  if ([a, b].some(isFalsy)) {
    throw new Error("Invalid numbers provided");
  }

  return a + b;  
}

sum(1, 3); // 4. Great!.
sum(null, 5); // ERROR: Invalid numbers provided.
sum(10, undefined); // ERROR: Invalid numbers provided.

Well, yeah, that works, however what if I want to know which number was the invalid one?

let sum = (a, b) => {
  if (isFalsy(a)) {
    throw new Error("The first addend was invalid");
  } else if (isFalsy(b)) {
    throw new Error("The second addend was invalid");
  }

  return a + b;  
}

sum(1, 3); // 4. Great!.
sum(null, 5); // ERROR: The first addend was invalid.
sum(10, undefined); // ERROR: The second addend was invalid.

Well this works perfectly! Now, let's see how that would work with an Either

/**
 * Either Approach
 */

let sum = (a, b) => {
  let eitherA = Either.unit(() => "The first addend was invalid", a);
  let eitherB = Either.unit(() => "The second addend was invalid", b);

  return eitherA.match({
    left: Either.left,
    right: addend1 =>
      eitherB.match({
        left: Either.left,
        right: addend2 => Either.right(addend1 + addend2)
      })
  });
}

sum(1, 3); // Right 4
sum(null, 5); // Left 'The first addend was invalid.'
sum(10, undefined); // Left 'The second addend was invalid.'

The Either path as well as the Maybe path, tries to lead you to doing what you have to do where you have to be doing it, is it Failure? Take the left path, and in the left path do work specific for a failure. Is it a Success? Take the right path, and in the right path do work specific for a success.