scala-type-variance

name	scala-type-variance
description	Translating Python TypeVar variance to Scala. Use this skill whenever you see T_co or T_contra in Python and need to write equivalent Scala traits or classes.

Scala Type Variance

• Python TypeVar("T_co", covariant=True) -> Scala [+T] (Covariant).
• Python TypeVar("T_contra", contravariant=True) -> Scala [-T] (Contravariant).
• Python TypeVar("T") -> Scala [T] (Invariant).

Invariance, Covariance, Contravariance rules in Scala

• Covariant types [+T] can only appear in covariant positions (e.g., return types of methods). If you need to accept T in a method parameter, use a lower type bound: def method[U >: T](val: U).
• Contravariant types [-T] can only appear in contravariant positions (e.g., arguments of methods). If you need to return T, use an upper type bound: def method[U <: T](): U.
• Mutable collections (like ArrayBuffer or mutable.Map) are invariant [T].
• Immutable collections (like List, Vector) are covariant [+T].
• Scala functions A => B are contravariant in A and covariant in B: [-A, +B].