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scala allows you to have abstract types
ok then i'm not sure when one would need a type to be existential as opposed to just a type parameter
for example, imagine
List[Algorithm]
and the in-out is the only thing that matters from the outside
if you had
Algorithm[State] you woudn't be able to put them in a List with a proper type
in this case through an abstract type
does that make any sense?
yeah
the answer is not that simple unfortunately
you need to distinguish between existential and universal types as a general concept (e.g. in type theory), and the programming language features that encode them
but in short, no, it's not an aesthetic thing
it sort of makes sense... coz when you define
trait MyTrait[T] you're saying MyTrait[T] exists for every T but if you say trait MyTrait {type T} you're making no such guarantees
it sort of makes sense... coz when you define
trait MyTrait[T]you're sayingMyTrait[T]exists for everyTbut if you saytrait MyTrait {type T}you're making no such guarantees
yes, you are making a different guarantee, that it exists a T for which MyTrait exist
those correspond to universal and existential quantification in logic
because at the heart of all this there's a very deep theoretical result which links type theory and logic
the
Curry-Howard isomorphism
for example in scala wildcard types are existentials,
forSome are existentials, plus there's the fact that abstract types are existential types, which you are seeing here
or is it the "typeclass trick" using implicits
No, I don't see any relevance to typeclasses and implicits here
so in some way you are encoding existentials using universals?
you can replace
Monoid[T] with anything that has a type parameter and explicit instances, and your question remains the same
second, you can look at existentials to be linked to information hiding