Scala Typeclass for methods with parameters

Question

I have tried to add methods like "multiply by a scalar" to Array[T] via a typeclass, to mimic numpy functionality. I have looked at an example, which works and adds a "show" method with no parameters to Array[T]. However, in my case, I find that I am unable to add methods with parameters, e.g. array.*(2.0).

Using Scala 2.12.6.

How can I make this code work?

package numpy

trait BehaveLikeNumpy[T] {
  def *(a: Array[T], x: T): Array[T]
}

object BehaveLikeNumpyInstances {

  def apply[T](implicit bln: BehaveLikeNumpy[T]): BehaveLikeNumpy[T] = bln

  object ops {
    def *[T: BehaveLikeNumpy](a: Array[T], x: T): Array[T] = BehaveLikeNumpyInstances[T].*(a, x)

    implicit class BehaveLikeNumpyOps[T: BehaveLikeNumpy](a: Array[T]) {
      def *(x: T) = BehaveLikeNumpyInstances[T].*(a, x)
    }
  }

  implicit val arrayTimes_Double = (a: Array[Double], x: Double) => a.map(y => y * x)
}

package some.pkg
import numpy.BehaveLikeNumpyInstances
import numpy.BehaveLikeNumpyInstances.ops._

val aaa: Array[Double] = (0 until 5).toArray.map(_.toDouble)
aaa.show // Works. See https://blog.scalac.io/2017/04/19/typeclasses-in-scala.html
val s1 = aaa.*(2.0)// Error: value * is not a member of Array[Double]
val s2 = aaa * 2.0 // Error: value * is not a member of Array[Double]

Dmytro Mitin · Accepted Answer · 2019-02-24 18:58:52Z

2

Firstly you lost type (for transformation of lambda to SAM trait)

implicit val arrayTimes_Double: BehaveLikeNumpy[Double] = 
  (a: Array[Double], x: Double) => a.map(y => y * x)

secondly you lost underscore

import numpy.BehaveLikeNumpyInstances._

answered Feb 24, 2019 at 18:58

Dmytro Mitin

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6 Comments

radumanolescu Over a year ago

With your proposed changes I get the same errors, plus: type mismatch; found: (Array[Double], Double) => Array[Double]; required: numpy.BehaveLikeNumpy[Double]

Dmytro Mitin Over a year ago

What is your Scala? This works in 2.12. In 2.11 you should write arrayTimes_Double: BehaveLikeNumpy[Double] = new BehaveLikeNumpy[Double]... manually instead of the lambda.

radumanolescu Over a year ago

Scala 2.12.6. Added it to the post (was not there initially).

Dmytro Mitin Over a year ago

No, you can just add def + to trait BehaveLikeNumpy, object ops and implicit class BehaveLikeNumpyOps, manually define implicit val arrayTimes_Double with new without lambda (since BehaveLikeNumpy is no longer a SAM trait) and with the same two imports both + and * should work.

Dmytro Mitin Over a year ago

Don't define two lambdas like implicit val arrayTimes_Double.

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