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  • Dec 15 2021 05:01
    som-snytt commented #12516
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    SethTisue edited #1312
Vinayak Pathak
@vinayakpathak
that is if I write val m: Model = new Model {...}?
Fabio Labella
@SystemFw
yes
Vinayak Pathak
@vinayakpathak
i see i see
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?
Fabio Labella
@SystemFw
nope
Vinayak Pathak
@vinayakpathak
def f[Model[_]] = ??? ?
Fabio Labella
@SystemFw
because you would have Model[State]
wait
the _ you are using there doesn't mean what you think it means
because scala is weird
like
I know what you mean
but your code there doesn't mean that
Vinayak Pathak
@vinayakpathak
i see!
Fabio Labella
@SystemFw
it means that you are taking a higher-kinded type Model[_]
or in other words
def f[Model[_]] = ???
is not the same
and in fact has nothing to do with
def f(model: Model[_]) = ???
this is what you mean
a wildcard type
which again, it's a feature that encodes existentials
Vinayak Pathak
@vinayakpathak
actually yes, that's what i meant indeed
isn't this saying that f takes a value of type Model but doesn't care what State is
Fabio Labella
@SystemFw
yes
it's introducing an existential again
which is clearer if you use the forSome thing (which however is going away)
def f(model: Model[A] forSome { type A })
Vinayak Pathak
@vinayakpathak
i see i see
Fabio Labella
@SystemFw
whereas def f[A](model: Model[A]) means forall type A
Vinayak Pathak
@vinayakpathak
right
ok so doesn't it then encode existential types?
Fabio Labella
@SystemFw
what's "it" now?
Vinayak Pathak
@vinayakpathak
umm def f(model: Model[_])
or the one using forSome
Fabio Labella
@SystemFw
yes it does
both do
but thats' because you are using specific features that introduce existentials
you are not using universals anymore
Vinayak Pathak
@vinayakpathak
oh ok
oh i see
Fabio Labella
@SystemFw

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

as I said above

Vinayak Pathak
@vinayakpathak
actually, we can't really access the State type inside f's definition in this case can we
Fabio Labella
@SystemFw
nope, because it's existential
for example
try taking two Model[_] and assigning the values of type State inside them to each other
and you'll see a weird error about _1and _2
this is how a lot of people come across existentials for the first time