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".
There isn't any consistency with what people mean when they say "HKT" these days. By most usages, type constructors *are* HKTs. Though, the way you've written it ("Option[_]") is actually just a proper type with kind *, albeit existential. "Option" would be the TC.
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you know you got the HKT when the compiler warns you about it :ppic.twitter.com/K6p85FBp9x
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Ha! If the compiler (just the REPL, right?) is being clear about this, now, then I should adopt its consistency. :)
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OK, s/Option[_]/Option/ then. But the problem I had with using "Option" is it's also what the companion object (term) is...
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Yep, Scala cheats away this ambiguity with context-aware syntax. I'm not sure how best to express the intent of Option *as a type* in prose.
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Yeah it seems basically a lost battle, tho I've seen
@larsr_h fight it: the incorrect usages outnumber the correct ones (and they appear even in the published literature). -
I'm not sure I understand. - Int, String, etc are nullary type constructors (*), aka proper types - Option is a unary type constructor (* -> *), as it accepts a proper type as an argument - Higher kinded type is the ability to take * -> * as a type argument, e.g. Functor[F[_]]
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