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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
as in "I have a Monoid for some type, but I dont know which type"
whereas when you get
Monoid[Int], you know that the Monoid is for Int
encoding existentials through universals in a simple way requires higher rank polymorphism (which I've talked about in this channel a few days ago), which scala doesn't have
go back to
Model
you have a Model for some
State, but you don't know which State
hence when you try to coerce it to Int, type mismatch
so existential types allow us to define values that have this information hiding feature
if you wanted to define a function that took a
Model without knowing its State, you could do that with universal types too no?
wait