Question

I am writing a Scala class that implements a 2-dimensional matrix of arbitrary objects. I need the class to be more specialized than nested pair of IndexedSeq objects, but extending a collections class is overkill, so I’m writing my own. In order to return the correct type from methods in my matrix class, I am using the implicit builder idiom, but at runtime I get a “could not find implicit value for parameter” error which I don’t understand.

A stripped-down version of my matrix class looks like this.

trait MatrixBuilder[V, M <: Matrix[V]] {
  def apply(values: IndexedSeq[IndexedSeq[V]]): M
}

abstract class Matrix[V](values: IndexedSeq[IndexedSeq[V]]) extends Function2[Int, Int, V] {

  def apply(row: Int, col: Int): V = values(row)(col)

  def set[M <: Matrix[V]](row: Int, col: Int, value: V)(implicit builder: MatrixBuilder[V, M]): M =
    builder(values.updated(row, values(row).updated(col, value)))
}

case class IntMatrix(values: IndexedSeq[IndexedSeq[Int]]) extends Matrix[Int](values)

object IntMatrix {
  def apply(n: Int) = new IntMatrix(IndexedSeq.fill(n, n)(0))

  implicit object IntMatrixBuilder extends MatrixBuilder[Int, IntMatrix] {
    def apply(values: IndexedSeq[IndexedSeq[Int]]) = IntMatrix(values)
  }
}

I want the set function to set the specified cell then return a new matrix of the correct type. So I expect IntMatrix(2).set(0,0,5) to return an IntMatrix object with zeros in all cells except (0,0), where it should have a 5. Instead I get the following error at runtime.

error: could not find implicit value for parameter builder: MatrixBuilder[Int,M]
    IntMatrix(2).set(0,0,5)

What am I doing wrong here?

---

As pedrofurla notes below, the code does work in the REPL if you first run the line import IntMatrix._. And looking at the collections documentation, there appear to be similar import statements in source code using builders. I tried adding one to my IntMatrix class.

case class IntMatrix(values: IndexedSeq[IndexedSeq[Int]]) extends Matrix[Int](values) {
    import IntMatrix._
}

But this has no effect. (In fact my IDE IntelliJ flags this as an unused import statement.)

For comparison I copied over the RNA sequence example from the collections documentation linked above verbatim. There the import RNA._ line is not marked as superfluous and all operations return the correct type. If the answer is that I need to add an import IntMatrix._, I can’t figure out where to put it.

Answer 1

This little code worked here:

scala> import IntMatrix._
import IntMatrix._

scala> IntMatrix(2).set(0,0,5)
res1: Mat.IntMatrix = <function2>

Implicit parameters are filled by compiler in the call site, so they have to be available in the scope set is being invoked.