scala/scala

A relaxed chat room about all things Scala (2 & 3 both). Beginner questions welcome. https://scala-lang.org/conduct/ applies

Jose C

@jmcardon

you can see it in the signature

Fabio Labella

@SystemFw

right, the important distinction here is not between type and type constructor

that doesn't matter

Jose C

@jmcardon

object None extends Option[Nothing]

Fabio Labella

@SystemFw

it's between type constructor and data constructor

the fact that Nothing/None have no arguments is not the point I'm making

let me put it this way

trepidacious

@trepidacious

@SystemFw I get that Nothing is different to a type or type constructor (at least approximately) yup.

Fabio Labella

@SystemFw

same for Just

yeah ok

Jose C

@jmcardon

@trepidacious what do you find weird about grasping re: type constructors?

trepidacious

@trepidacious

@SystemFw Yes. But it's still the case that to have a functor you need a type constructor rather than a type? I get that Just and Nothing fail even BEFORE that by not being a type OR a type constructor :)

Fabio Labella

@SystemFw

(btw, the distinction between type and type constructor is not really useful imho except in conversational talking, they're all types afterall, and we have kinds to talk about them precisely)

a Functor needs to be over a type of kind * -> *

that's what you need

trepidacious

@trepidacious

@SystemFw Is * -> * not the same as a type constructor? I thought they were just different terms for the same thing?

Fabio Labella

@SystemFw

yeah, but type constructor is not as precise

think Either

trepidacious

@trepidacious

@jmcardon I think I'm ok on that, we just ended up talking about two things at once, I didn't know about type constructors versus data constructors though

Fabio Labella

@SystemFw

that's also a type constructor

but can't be a functor

Either a can

Jose C

@jmcardon

b-b-b-b-bifunctor!

trepidacious

@trepidacious

Ah because it has two type paramters

Fabio Labella

@SystemFw

so I'd recommend just talking in terms of kinds

it's way more precise

trepidacious

@trepidacious

Is that * -> * -> * then?

Fabio Labella

@SystemFw

yeah

and in haskell types and "type constructors" have the same representation anyway

trepidacious

@trepidacious

How do I pronounce *->*?

Fabio Labella

@SystemFw

the term "type constructor" is imho only useful when explaining the difference with "data constructors"

star to star

that's how I do it

Rich

@Rich2

Could you implement Option in terms of Either?

type Option[A] = Either[Unit, A]
type Some[A] = Right[Unit, A]
val None = Left[Unit, Nothing](Unit)

Fabio Labella

@SystemFw

you can also say Type to Type but it's more confusing since you are saying type a lot as well

trepidacious

@trepidacious

Makes sense. So that is kind of a single parameter function on types?

Fabio Labella

@SystemFw

yeah, pretty much

with the usual caveats wrt to currying

Jose C

@jmcardon

star to star :P

@Richtype yeah

trepidacious

@trepidacious

I should read up on the data constructor thing, but approximately Just and Nothing are ways of getting a value of type Maybe, but that does not make them types themselves?

Jose C

@jmcardon

you can

Option is essentially Either with less information

Fabio Labella

@SystemFw

@trepidacious yes

Jose C

@jmcardon

you discard the information on the left parameter

Fabio Labella

@SystemFw

Some and Nothing do two things

Jose C

@jmcardon

and you can implement option

trepidacious

@trepidacious

Sorry of type Maybe a I guess

Fabio Labella

@SystemFw

one) they construct values of type Maybe a
two) they are runtime tags that are used to do runtime dispatch (pattern matching)

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