run2-scala-variance

name	run2_scala-variance
description	Expressing variance annotations in Scala generics when translating from Python TypeVar covariant/contravariant types.

Variance in Scala

Mapping from Python TypeVar

• TypeVar("T_co", covariant=True) -> [+T] (covariant)
• TypeVar("T_contra", contravariant=True) -> [-T] (contravariant)
• TypeVar("T") -> [T] (invariant)

Covariant Container

class TokenContainer[+T](items: Seq[T]) {
  // Can return T but not accept T as parameter in public methods
  def getAll: Vector[T] = items.toVector
}

Contravariant Sink

class TokenSink[-T] {
  // Can accept T but not return T
  private val received = mutable.ListBuffer[Any]()
  def receive(item: T): Unit = received += item
  def drain(): List[Any] = { ... }
}

Invariant Handler

class BivariantHandler[T](default: T) {
  // Both accepts and returns T
  def get: T = ...
  def set(value: T): Unit = ...
}