Function in generic class

Question

I have class Seq and I want to have method "multy" that adds number to List factors, but I want, that only Seq of Number types will have this method, for this example, val numbers should work with multy, and val strings shouldn't.

import scala.collection.mutable.ListBuffer object Main extends App{ val strings = new Seq("f", "d", "a") val numbers = new Seq(1,5,4,2) val strings2 = new Seq("c", "b") numbers.multy(5) strings.multy(5) val strings3 = strings2.concat(strings) println(strings3) println(numbers) } class Seq[T : Ordering] (initialElems: T*) { override def toString: String = elems.toString val factors = ListBuffer[Number](1) val elems = initialElems.sorted def concat(a:Seq[T]) = new Seq(a.elems ++ this.elems:_*) def multy[T <: Number](a:Number) = { factors += a; } }

scala.Int, scala.Double, etc are not subclasses of Number — Mateusz Kubuszok
– Mateusz Kubuszok, Commented May 7, 2020 at 11:42

Mateusz Kubuszok · Accepted Answer · 2020-05-07 12:16:39Z

If you want scala.Int, scala.Long, scala.Double, etc to not be supported - because they don't extend Number - you can define method as

def multy(a: Number)(implicit ev: T <:< Number): Unit = { factors += a }

However, considering that under the hood they could be extending Number (if the compiler, decides that it should compile them as objects and not as primitives), usage of a type class would work better:

trait IsNumber[T] { def asNumber(value: T): Number } object IsNumber { implicit def numbersAreNumbers[T <: Number]: IsNumber[T] = t => t implicit val intIsNumber: IsNumber[Int] = i => (i: Number) ... // the same for Long, Short, etc }

which could be used in multy as evidence

def multy(a: Number)(implicit ev: IsNumber[T]): Unit = { factors += a }

and as normal type class in code that would need that knowledge for something:

def doSomethingWithElements(implicit isNumber: IsNumber[T]) = { elems.map(isNumber.asNumber).toList // List[Number] }

This would support both java.lang.* Numbers as well as Scala's numbers definitions.

If I need only scala Int, Double, Float to be for multy, what can I use instead AnyVal (because it contains boolean for example) def multy(a: Number)(implicit ev: T <:< AnyVal):
Numeric doesn't expose method of casting to java.lang.Number that OP might want. It allows casting directly .toLong, .toDouble etc, which could change the value underneath. If this Number x Number semantics is not needed (I assume it is because factors are Numbers) and OP would be OK with factory: List[T] (empty if T is not Numeric) and appending to it only if T is Numeric, then yes, Numeric would be sufficient.

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Function in generic class

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