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    SethTisue commented #751
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    SethTisue on 2.13.x

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    julienrf commented #2137
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Fabio Labella
@SystemFw
no, this is a different thing
Vinayak Pathak
@vinayakpathak
okay
Fabio Labella
@SystemFw
first of all, your example has not much to do with typeclasses or implicits
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
Vinayak Pathak
@vinayakpathak
true
Fabio Labella
@SystemFw
matter of fact
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
Vinayak Pathak
@vinayakpathak
i see
Fabio Labella
@SystemFw
you could simulate it in this case, but then again abstract types already give you a way of encoding existential quantification, so...
Vinayak Pathak
@vinayakpathak
i'm thinking about your statement: "I have a Monoid for some type, but I dont know which type"
Fabio Labella
@SystemFw
yeah, that doesn't hold
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
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