CODEWITHSUNDEEP
scalaimplicits
WestCoastProjects
WestCoastProjects

Reputation: 63062

Arranging implicits precedence in Scala

Consider the following (working) snippet that defines bidirectional implicit conversions between the DenseVector and Arrays:

import scala.reflect.ClassTag
import org.apache.spark.mllib.linalg.Vectors
import breeze.linalg.{DenseVector => BDV}

  implicit def bdvToSeq[T](vect: BDV[T])(implicit ct: ClassTag[T]): Seq[T] = vect.toArray.toSeq

  implicit def arrayToVect(darr: Array[Double]): BDV[Double]
   = new BDV(darr)
  implicit def bdvToArray[T](vect: BDV[T])(implicit ct: ClassTag[T]): Array[T] = vect.toArray

  def add(v1: BDV[Double], v2: BDV[Double]) =
    v1.zip(v2).map { x => x._1 + x._2}

Let's try it out:

scala> arr.allVisitableIndicesActive   // Array to DenseVector implicit conversion
res4: Boolean = true

scala> val d = new BDV(Array(77.7, 88.8, 99.9))
d: breeze.linalg.DenseVector[Double] = DenseVector(77.7, 88.8, 99.9)

Now the other direction:

scala> def myArrMethod(arr: Array[Double]) = println(arr.mkString(","))
myArrMethod: (arr: Array[Double])Unit

scala> myArrMethod(d)   // DenseVector to array conversion
77.7,88.8,99.9

The following also works: notice the darr.toSeq

  def norm(darr: Array[Double]): Double = {
    Math.sqrt(darr.toSeq.foldLeft(0.0) { case (sum, dval) => sum + Math.pow(dval, 2)})
  }

However if we omit the .toSeq then we have implicits collision:

scala>   def norm(darr: Array[Double]): Double = {
        Math.sqrt(darr.foldLeft(0.0) { case (sum, dval) => sum + Math.pow(dval, 2)})
       }
<console>:34: error: type mismatch;
 found   : darr.type (with underlying type Array[Double])
 required: ?{def foldLeft(x$1: ? >: Double(0.0)): ?}
Note that implicit conversions are not applicable because they are ambiguous:
 both method doubleArrayOps in object Predef of type (xs: Array[Double])scala.collection.mutable.ArrayOps[Double]
 and method arrayToVect of type (darr: Array[Double])breeze.linalg.DenseVector[Double]
 are possible conversion functions from darr.type to ?{def foldLeft(x$1: ? >: Double(0.0)): ?}
           Math.sqrt(darr.foldLeft(0.0) { case (sum, dval) => sum + Math.pow(dval, 2)})
                     ^
<console>:34: error: value foldLeft is not a member of Array[Double]
           Math.sqrt(darr.foldLeft(0.0) { case (sum, dval) => sum + Math.pow(dval, 2)})

I would however prefer not to put the ".toSeq" explicitly.

So my question is: how to set the proper precedence so that the Predef.toSeq is the clear winner - always tried before the custom lower priority implicits defined above?

Upvotes: 0

Views: 245

Answers (1)

Eugene Zhulenev
Eugene Zhulenev

Reputation: 9734

You can extract all your implicit conversions into object, and import them only where needed

trait BDVImplicits {
  implicit def bdvToSeq[T](vect: BDV[T])(implicit ct: ClassTag[T]): Seq[T] =
    vect.toArray.toSeq

  implicit def arrayToVect(darr: Array[Double]): BDV[Double] =
    new BDV(darr)

  implicit def bdvToArray[T](vect: BDV[T])(implicit ct: ClassTag[T]): Array[T] =
    vect.toArray
}

object BDVImplicits extends BDVImplicits

If you'll place your 'norm' method in Utils for example, and don't import implicit you'll be good. You'll just need to import BDVImplicits._ in class where you call your Util methods.

I think it's simplest workaround for this problem.

Upvotes: 1

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