Scala3 extensions for basic types and overriding == - scala

Question

Learning the Scala3 extension and CanEqual concepts, but finding difficulty in extending certain features of an Int.
In the following example I am easily able to add >= functionality to Int to compare it to a RationalNumber case class, but unable to modify the behavior of ==. (note 1~2 is the same as RationalNumber(1,2)).
The problem seems to be tied in with basic AnyVal types and how Scala passes off to Java to handle equals and ==.
case class RationalNumber(val n: Int, val d: Int):
def >=(that:RationalNumber) = this.num * that.den >= that.num * this.den
//... other comparisons hidden (note not using Ordered for clarity)
private def gcd(a: Int, b: Int): Int = if (b == 0) a else gcd(b, a % b)
val sign = if (n<0 ^ d<0) -1 else 1
private val (an, ad) = (math.abs(n), math.abs(d))
val num = sign * (an / gcd(an, ad))
val den = if(an == 0) 1 else ad / gcd(an, ad)
override def equals (that: Any): Boolean =
that match
case t: RationalNumber => t.den == den && t.canEqual(this) && t.num == num
case t: Int => equals(RationalNumber(t,1))
case _ => false
override lazy val toString = s"$num/$den"
object RationalNumber:
def apply (r: Int): RationalNumber = new RationalNumber(r, 1)
import scala.language.implicitConversions
implicit def intToRat (i: Int): RationalNumber = i ~ 1
given CanEqual[RationalNumber, Int] = CanEqual.derived
given CanEqual[Int, RationalNumber] = CanEqual.derived
extension (i: Int)
def ~(that: Int) = new RationalNumber(i, that)
def >=(that: RationalNumber) = i ~ 1 >= that
def equals (that: AnyVal) : Boolean =
println("this never runs")
that match
case t: RationalNumber => t.den == 1 && t.num == i
case _ => i == that
def ==(that: RationalNumber) =
println ("this never runs")
i~1 == that
object Main:
#main def run =
import RationalNumber._
val one = 1 ~ 1
val a = 1 == one // never runs extension ==
val b = one == 1
val c = 1 >= one
val d = one >= 1
val ans = (a,b,c,d) // (false, true, true, true)
println(ans)

Answer 1

Extension methods are tried only if a qualifying method of the same name does not already exist. Hence since at least the following qualifying == is already defined on Int
def ==(arg0: Any): Boolean
it will not call your extension. If you change the name to say === then it would work
def ===(that: RationalNumber)
You could force implicit conversion with type ascription (1: RationalNumber) == one if you want. (Implicit conversions are discouraged).
Try extending ScalaNumericConversions which in turn extends ScalaNumber
case class RationalNumber(val n: Int, val d: Int) extends ScalaNumericConversions {
def intValue: Int = ???
def longValue: Long = ???
def floatValue: Float = ???
def doubleValue: Double = ???
def isWhole: Boolean = false
def underlying = this
...
override def equals (that: Any): Boolean = {
that match {
case t: RationalNumber => t.den == den && t.canEqual(this) && t.num == num
case t: Int => equals(RationalNumber(t,1))
case _ => false
}
}
}
so now Scala will eventually call BoxesRuntime#equalsNumNum
public static boolean equalsNumNum(java.lang.Number xn, java.lang.Number yn) {
...
if ((yn instanceof ScalaNumber) && !(xn instanceof ScalaNumber))
return yn.equals(xn);
}
...
which note flips the order of arguments and hence will call RationalNumber#equals, so in effect
1 == one
becomes
one.equals(1)
Found this approach by looking at the :javap - in REPL for 1 == BigInt(1)
30: invokestatic #54 // Method scala/runtime/BoxesRunTime.equals:(Ljava/lang/Object;Ljava/lang/Object;)Z
and then following trail laid out by BoxesRunTime.equals