The Either type provides is used to represent values that can be one of two possibilities. For example, Either Int Number can be used in a place where either integers or floating point numbers are acceptable.
A common use for this type is error handling, where by convention the Left constructor carries error messages and the Right constructor carries expected values (“Right is right”). This convention is used as the Functor, Applicative, Monad, etc. instances all operate on the Right part of the Either.
bower install purescript-either