A data-type like Either but with differing properties and type-class instances.
Library support is provided for this different representation, include
lens-related functions for converting between each and abstracting over their
Validation data type is isomorphic to
Either, but has an instance
Applicative that accumulates on the error side. That is to say, if two
(or more) errors are encountered, they are appended using a
As a consequence of this
Applicative instance, there is no corresponding
Validation is an example of, "An applicative
functor that is not a monad."