So, to denote that a function returns no value we use `Unit`. How should I denote that a parameter t...

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[Marc Knaup]

So, to denote that a function returns no value we use Unit.
How should I denote that a parameter takes no value? Unit or Nothing??

That’s relevant in a generic context like Visitor<Result, Data> where some visitors need no input and hence use Unit or Nothing? as Data.

[Scott Christopher]

The Unit type has one single possible instance (Unit), while Nothing has no value instances. With that in mind, Unit -> A is a function that doesn't require anything interesting to produce an A, while Nothing -> A is a function that can never be called because it's not possible to create the argument instance to call it with.

[Marc Knaup]

The choice is between Unit and Nothing?, i.e. an optional Nothing.
Both have exactly one value: Unit has Unit and Nothing? has null.

[Scott Christopher]

Sorry, I missed the “?”. In that case, if I understand the intent correctly, I’d recommend Unit because “Nothing?” can only ever be null and so the function is only ever called with null. I find Unit conveys the same intent more clearly.

[Marc Knaup]

Yeah I thought so too.

I’m surprised that Kotlin compiler code uses Nothing? in that case so I think it’s a topic worth discussing 😄

[Marc Knaup]

https://github.com/JetBrains/kotlin/blob/0e4e5ac287c3836ca4434c2c6f626b9145c284e5/compiler/fir/tree/gen/org/jetbrains/kotlin/fir/visitors/FirVisitorVoid.kt#L128

[Scott Christopher]

Interesting, it may be an optimisation?

[Marc Knaup]

Maybe.
Or maybe because Unit means there’s a value (Unit) and null better represents the absence of a value.

[Scott Christopher]

I’d personally find Unit to better represent the absence of any meaningful value, but... ¯\_(ツ)_/¯

[Marc Knaup]

yeah ¯\_(ツ)_/¯