A function wrapping calls to the two functions in an
returning the result of the first function if it is truth-y and the result
of the second function otherwise. Note that this is short-circuited,
meaning that the second function will not be invoked if the first returns a
In addition to functions,
R.either also accepts any fantasy-land compatible
a function that applies its arguments to `f` and `g` and `||`s their outputs together.