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
} val square = Square(4)
println(square.height)
println(square.width)
println(square.size)
println(square.area)
println(square.perimeter)
println(square.sides)4.0
4.0
4.0
0.0
0.0
4
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 * widthAnother 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 * widthDue to the immutability of the traits and classes, they will always be the correct value as neither height nor width will be subsequently updated.
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”.
flatMap takes f that returns Sum
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
This (section of this) article explains it well but it's in Haskell
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.flatMap out, it should all fit
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
1 and -1, they keep switching. Roughly speaking, functions of functions is the multiplication
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!
List(1).withFilter(_ > 0)
val res63: scala.collection.WithFilter[Int,[_]List[_]] = scala.collection.IterableOps$WithFilter@cf76c0bdef something(list: List[_]), then the [_] is a wildcard, meaning you can pass a list value whose elements are any type.
[_] there means "i don't care what the type is, so i'm going to forget its name", just like _ in places like pattern matching means "i don't care what this is, i'm not even going to name it")
