How to use Numeric[T] to represent zero of any numeric type

user_1650448 0
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I have a piece of Scala code that I'd like to make more generic. I know that Numeric[T] exists, but I'm not sure how to use it.

  def sumMaps[A](m1: Map[A, Long], m2: Map[A, Long]): Map[A, Long] = {
    m1 ++ m2.map { case (k, v) => k -> (v + m1.getOrElse(k, 0L)) }
  }

  def sumMaps[A](m1: Map[A, Int], m2: Map[A, Int]): Map[A, Int] = {
    m1 ++ m2.map { case (k, v) => k -> (v + m1.getOrElse(k, 0)) }
  }

  def sumMaps[A](m1: Map[A, Double], m2: Map[A, Double]): Map[A, Double] = {
    m1 ++ m2.map { case (k, v) => k -> (v + m1.getOrElse(k, 0)) }
  }

I'd like to write something like this (just once), where the zero value gets auto-converted to a B type.

  def sumMaps[A, B: Numeric[?]](m1: Map[A, B], m2: Map[A, B]): Map[A, B] = {
    m1 ++ m2.map { case (k, v) => k -> (v + m1.getOrElse(k, 0)) }
  }
zero typeclass type-inference scala generics
1 Answers

Try

def sumMaps[A, B](m1: Map[A, B], m2: Map[A, B])(implicit num: Numeric[B]): Map[A, B] = {
  m1 ++ m2.map { case (k, v) => k -> num.plus(v, m1.getOrElse(k, num.zero)) }
}

or

import Numeric.Implicits._
def sumMaps[A, B](m1: Map[A, B], m2: Map[A, B])(implicit num: Numeric[B]): Map[A, B] = {
  m1 ++ m2.map { case (k, v) => k -> (v + m1.getOrElse(k, num.zero)) }
}

or

def sumMaps[A, B: Numeric](m1: Map[A, B], m2: Map[A, B]): Map[A, B] = {
  val num = implicitly[Numeric[B]]
  import num._
  m1 ++ m2.map { case (k, v) => k -> (v + m1.getOrElse(k, zero)) }
}

The imports provide implicit conversion infixNumericOps

m1 ++ m2.map { case (k, v) => k -> (infixNumericOps(v) + m1.getOrElse(k, num.zero)) }

so we do not have to explicitly use Numeric.plus like in the first example.

user_52050220
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