scala - What's the purpose of null in X >: Null <: Y? -


for example:

abstract type classsymbol >: null <: universe.typesymbol universe.classsymbolapi

this found here. realize there description of does, can't make sense of syntax standpoint.

what's purpose of null? looks might kind of default surmise because see kind of syntax types, , think null can returns type without program breaking. abstract type, doesn't have parameter can specify later, how work abstract type?

null type specifies can null. it's not supertype, strictly speaking, because can't anything null except pass around , check that, yep, it's still null. in way it's nothing, except can't instance of nothing pass around.

now, 1 say, "well, anyref null", how think of instance null itself? null can fill in pretty anything, which, type, means must subtype of can null! , that's null is: subtype of can null.

wait, wait, say. can't actually have subtype of everything--that means null have every possible functionality! if think it, null does promise every type of functionality anyref has--but they're empty promises, , whenever try use functionality nullpointerexception instead.

now, question is, why make type >: null (meaning null valid value it) instead of <: anyref should imply same thing?

well, turns out scala doesn't model things way, leave open possibility have anyref absolutely sure not ever possibly null. asking <: anyref ("i subclass of anyref") doesn't mean same thing theoretically, though in practice absolutely-not-null-anyrefs not implemented. (also, any awkward if null not explicit type.)

so type foo >: null <: bar means explicitly admit null can fill in instances of possible type of foo, , can write val foo: foo = null.


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