Yesterday I did a presentation to the team titled "never use Boolean" and today I got two pull requests with Option[Unit] - urgh, well played!! 
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They’re inferior to bespoke enumerated types with semantically relevant constructors. How many times have you seen a function argument where the meaning of a boolean value was a bit ambiguous, or where you had to guess based on the argument name whether to negate or not?
I think it's more like, in practical terms; not: isUpperCase :: Char -> Bool instead: upperCase :: Prism' Char UpperCaseChar and if you design your code this way, and although there now exists a function, Char -> Bool from the prism, it will rarely (never?) be used.
