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  • 01:06

    SethTisue on 2.12.x

    2.12: advance project SHAs (#14… (compare)

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    SethTisue closed #1463
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Fabio Labella
@SystemFw
do you need State to be existential there, since you are already making the whole Model existential in Algorithm?
Vinayak Pathak
@vinayakpathak
i'm not sure what existential means
Fabio Labella
@SystemFw

ummm why is Model valid

Because scala allows you to have abstract (existential) types

well, replace that word with "abstract"
scala allows you to have abstract types
Vinayak Pathak
@vinayakpathak
oh i see ok
ok then i'm not sure when one would need a type to be existential as opposed to just a type parameter
Fabio Labella
@SystemFw
the two things, type parameters and abstract types (universals and existentials) are two different ways to enforce information boudaries
jonathan edwards
@_jedw_twitter
@abdheshkumar Thanks man, I guess i wasn’t reading enough of the page.
Fabio Labella
@SystemFw
well, one use case is to hide the specific type used there
for example, imagine List[Algorithm]
Vinayak Pathak
@vinayakpathak
i see
Fabio Labella
@SystemFw
assume that each algorithm has an input, output and internal state
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
Vinayak Pathak
@vinayakpathak
hmm i see
Fabio Labella
@SystemFw
but since in my hypothetical example the State is purely internal, you hide it by making it existential
in this case through an abstract type
does that make any sense?
Vinayak Pathak
@vinayakpathak
ok got it
yeah
Fabio Labella
@SystemFw
cool
Vinayak Pathak
@vinayakpathak
but is it an aesthetic thing or do their exist things we couldn't do if existential types didn't exist
Fabio Labella
@SystemFw
that's a really good question
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
Vinayak Pathak
@vinayakpathak
ahh so universal type being another name for type parameters?
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
Fabio Labella
@SystemFw
it's a fundamental distinction conceptually. However it is possible to encode existentials through universals with a trick. That's why I'm not sure how to answer your question without being either imprecise or confusing :P

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

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
Vinayak Pathak
@vinayakpathak
yeah... i've heard about that link several times
Fabio Labella
@SystemFw
now, as I said the type theory concepts and the mapping of such concepts to specific programming language feature vary
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
Vinayak Pathak
@vinayakpathak
makes sense... so existential vs universal types are a type theory concept
Fabio Labella
@SystemFw
yes
Vinayak Pathak
@vinayakpathak
fascinating
Fabio Labella
@SystemFw
indeed :)
Vinayak Pathak
@vinayakpathak
is forSome the trick that lets you encode existential types using universals?
Fabio Labella
@SystemFw
no, it's a native feature for existentials
Vinayak Pathak
@vinayakpathak
or is it the "typeclass trick" using implicits
Fabio Labella
@SystemFw
which is half-broken, and it's going away

or is it the "typeclass trick" using implicits

No, I don't see any relevance to typeclasses and implicits here

Vinayak Pathak
@vinayakpathak
well when you define a typeclass such as trait Monoid[T] you are saying that a Monoid[T] exists for every T however, the way you implement it using implicits, it is guaranteed that it only exists for the T's for which you have defined a typeclass instance
so in some way you are encoding existentials using universals?
Fabio Labella
@SystemFw
no, this is a different thing
Vinayak Pathak
@vinayakpathak
okay