Scala generics variance

Generics and Variance in Scala

Python's typing module provides TypeVar to represent generics, including covariant and contravariant parameters. Scala has built-in support for declaration-site variance.

Type Parameters

• Invariant: class MyClass[A] (Must be exact type, like Python TypeVar("T"))
• Covariant: class MyClass[+A] (Can be subtype, like Python TypeVar("T_co", covariant=True))
• Contravariant: class MyClass[-A] (Can be supertype, like Python TypeVar("T_contra", contravariant=True))

Example: Covariant Container

Python:

T_co = TypeVar("T_co", covariant=True)
class Container(Generic[T_co]):
    def __init__(self, items: Sequence[T_co]):
        self._items = tuple(items)
    def get_all(self) -> tuple[T_co, ...]:
        return self._items

Scala:

class Container[+A](items: Seq[A]) {
  private val _items: Vector[A] = items.toVector
  def getAll: Vector[A] = _items
}

Protocols vs Traits

Python Protocol allows duck-typing, but Scala requires nominal typing via trait. Python:

class Tokenizable(Protocol):
    def to_token(self) -> str: ...

Scala:

trait Tokenizable {
  def toToken: String
}

If an object needs to implement this, it must explicitly extend Tokenizable.