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##### Activity
Noel Welsh
@noelwelsh
nafg
@nafg
I think you're thinking of case classes
The tuple limit is only dropped in dotty
Noel Welsh
@noelwelsh
Gotcha
Sachin Kukreja
@sk364
I was just wondering, if there was some logical analysis behind choosing the number 22, which I am unable to see.
I am guessing the architecture of the tuples is such that more than 22 elements is too much memory?
Fabio Labella
@SystemFw
no, it's arbitrary
the way tuples and functions are encoded are with a bunch traits, Tuple1, Tuple2... and so on
謝宇恆
@xieyuheng
22 is the secret seed for the finial answer, 22 + 22 - 2 = 42
Noel Welsh
@noelwelsh
:boom:
Shouvik Roy
@royshouvik
Just wanted to drop by and thank the authors and everyone behind the Creative Scala book. I am really enjoying the book :clap:
Visar Zejnullahu
@visar
Hi guys
I'm doing Excercise 4.1.4.3 from Essential Scala
case class Square(size: Double) extends Rectangular {
val height: Double = size
val width: Double = size
}
trait Rectangular extends Shape {
def height: Double
def width: Double
val sides: Int = 4
override val perimeter: Double = 2 * (height + width)
override val area: Double = height * width
}
trait Shape {
def sides: Int
def perimeter: Double
def area: Double
}
and in my Main I have this
 val square = Square(4)

println(square.height)
println(square.width)
println(square.size)
println(square.area)
println(square.perimeter)
println(square.sides)
but I'm getting the following results
4.0
4.0
4.0
0.0
0.0
4
I tried debugging
Visar Zejnullahu
@visar
and I can see that perimeter and area are set to 0
Any idea why?
SuprF1y
@SuprF1y

perimeter and area should be defined with def as methods rather than as vals

  override def perimeter: Double = 2 * (height + width)
override def area: Double = height * width

Another way to do it is declare them lazy. Then they will only be initialised upon first use

 override lazy val perimeter: Double = 2 * (height + width)
override lazy val area: Double = height * width

Due to the immutability of the traits and classes, they will always be the correct value as neither height nor width will be subsequently updated.

Marjan Mubarok
@MarjanMubarok
Hello, I plan on doing a presentation on the different ways Scala supports concurrency. Does anyone know of any good books on the topic?
Charis Loveland
@charislove
Hi, here are some edits for the Atom install documentation. Thank you very much for the hard work on this! At https://www.creativescala.org/creative-scala.html#installing-terminal-software-and-a-text-editors, change "Install Scala support inside Atom: Settings > Install > language-scala" to the following: "Go to atom.io and download and install the Atom text editor. Open Atom and go to the Settings tab. Then click on "+ Install" and type "language-scala" and install."
Noel Welsh
@noelwelsh
Thanks @charislove !
georgreen
@georgreen

Works

sealed trait Sum[+A, +B] {
def flatMap[AA >: A, C](f: B => Sum[AA, C]): Sum[AA, C] =
this match {
case Failure(v) => Failure(v)
case Success(v) => f(v)
}
}
final case class Failure[A](value: A) extends Sum[A, Nothing]
final case class Success[B](value: B) extends Sum[Nothing, B]

Doesn't Work

sealed trait Sum[+A, +B] {
def flatMap[C](f: B => Sum[A, C]): Sum[A, C] =
this match {
case Failure(v) => Failure(v)
case Success(v) => f(v)
}
}
final case class Failure[A](value: A) extends Sum[A, Nothing]
final case class Success[B](value: B) extends Sum[Nothing, B]

I don't get it. If I take f: B => Sum[A, C] and construct it from trait Function1[-I, +O]{ def apply(i: I): O} it seems B is the contravariant argument while Sum[A, C] is the covariant term in this function. How does it end up that A is the contravariant term also, what does this mean 👇 ?

"It is declared with type B => Sum[A, C] and thus a supertype is covariant in B and contravariant in A and C."

I expected it to be B is the contraviant term, why? it's the argument to function f,meaning it's B that has to be type bounded not A and Sum[A, C] to be covariant why? it's the result type of f. But I'm clearly wrong, what I'm I missing here?

I didn't really get the argument for contravariant terms in a function why supertypes?

Source:

"Back to flatMap, the function f is a parameter, and thus in a contravariant position. This means we accept supertypes of f. It is declared with type B => Sum[A, C] and thus a supertype is covariant in B and contravariant in A and C. B is declared as covariant, so that is fine. C is invariant, so that is fine as well. A on the other hand is covariant but in a contravariant position. Thus we have to apply the same solution we did for Box above."

Excerpt From: Noel Welsh and Dave Gurnell. “Essential Scala”.

Fabio Labella
@SystemFw
I don't really have time for a full explanation, but you can look up "negative and positive position"
basically when you nest functions, things switch from positive to negative to positive to negative (and so does their variance)
in this case: flatMap takes f that returns Sum
georgreen
@georgreen
@SystemFw, thank you.
Fabio Labella
@SystemFw

f: B => Sum[A, C] and construct it from trait Function1[-I, +O]{ def apply(i: I): O} it seems B is the contravariant argument while Sum[A, C] is the covariant term in this function

Right, but now if you write flatMap you f is in turn in negative position, so all the variances switch

anyway, I would fully agree it's very confusing
this is tl;dr
Positive position: the type variable is the result/output/range/codomain of the function
Negative position: the type variable is the argument/input/domain of the function
When a type variable appears in positive position, the data type is covariant with that variable.
When the variable appears in negative position, the data type is contravariant with that variable.
so if you apply these rules and try to work the type of flatMap out, it should all fit
in f, Sum is in positive position (return type)
flatMap is a function that takes f, so all of f is in negative position, which makes B have positive position and Sum have negative position
which makes it contravariant
the least crap intuition is multiplying numbers by 1 and -1, they keep switching. Roughly speaking, functions of functions is the multiplication
@msali123
Hi
I'm new here. Is there any way to host a project built with scala. On github pages or etc?
nano
@mr-nano
Hey folks. Glad to find this channel. Wish for everyone's safety in this crazy virus time!
Noel Welsh
@noelwelsh
:+1:
Josué
@eusojk
Hi all,
I just started my Scala/FP learning journey with Creative Scala. Very fantastic and engaging so far :raised_hands: !
For some reason, the images (figures) on the website don't render for me. I've tried multiple browsers with no luck. Has anyone experienced this by any chance? Thanks in advance!
hi, what does the [_] before the List in the following refer to?
List(1).withFilter(_ > 0)
val res63: scala.collection.WithFilter[Int,[_]List[_]] = scala.collection.IterableOps\$WithFilter@cf76c0b