scala-generics-variance

name	scala-generics-variance
description	Guide for translating Python TypeVars and generics to Scala covariant and contravariant types.

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.