Because Set is invariant in the specified Scala library, even returning Set[U] would place the class's covariant type parameter in an invariant position. Introduce a fresh widened type for each lifting.
class CoPartialFnExt[T,+U](data: List[(T,U)])
    extends CoPartialFn[T,U](data) {

  def forwardLift[V >: U](s: Set[T]): Set[V] = {
    val ys: List[V] = this.data.collect {
      case (x,y) if s.contains(x) => y
    }
    ys.toSet
  }

  def backwardLift[V >: U](s: Set[V]): Set[T] =
    this.data.collect {
      case (x,y) if s.contains(y) => x
    }.toSet
}
The lower type bound gives , so stored output values can be widened into List[V] or passed to s.contains. Both exposed Set types use invariant method type parameters, not the class's U. Calling forwardLift[U] or backwardLift[U] when the concrete output type is known recovers the exact output universe; a broader admissible universe is also safe.
For a partial function, the forward formula is understood only for inputs in its domain of a function. The implementations therefore compute
(1)
The conversion to a set removes repeated images. Inputs not in the domain of a function contribute nothing to either lifting.

Ancestors (11)

  1. 6
  2. Imperative programming
  3. 2015
  4. Past exam of the year 1 of the computer science course of the University of Oxford
  5. Year 1 of the computer science course of the University of Oxford
  6. Computer science course of the University of Oxford
  7. Undergraduate course of the University of Oxford
  8. Course of the University of Oxford
  9. University of Oxford
  10. List of universities
  11. Home

View article source

Discussion (0) New discussion

There are no discussions about this article yet.

Articles by others on the same topic (0)

There are currently no matching articles.
See all articles in the same topic Create my own version