Mastering JavaScript Functional Programming - Second Edition

Handling alternatives – the Either monad

Knowing that a value was missing may be enough in some cases, but in others, you'll want to be able to provide an explanation. We can get such an explanation if we use a different functor, which will take one of two possible values: one associated with a problem, error, or failure, and another associated with normal execution, or success: 

• A left value, which should be null, but if present then it represents some kind of special value (for example, an error message or a thrown exception) that cannot be mapped over
• A right value, which represents the normal value of the functor and can be mapped over

We can construct this monad in a similar way to what we did for Maybe (actually, the added operations ...