TIL Option[_] isn't a higher-kinded type. It's just a type constructor. Higher-kinded types are types that abstract over type constructors (so X[F[_]]). As known as 1st order & higher-order abstraction. But best refer to the latter as "type constructor polymorphism".
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you know you got the HKT when the compiler warns you about it :ppic.twitter.com/K6p85FBp9x
Ha! If the compiler (just the REPL, right?) is being clear about this, now, then I should adopt its consistency. :)