Universal Types Want It All, Existential Types Play It Cool: Covariance and Contravariance...

Universal quantifiers are covariant in their bound and allow for safe substitution with subtypes, enabling a more general type to represent specific types when accessed.

In Scala, the use of upper bounds in type parameters illustrates universal quantification where a List[+A] demonstrates how covariance allows for safer type substitutions across hierarchies.

Existential quantifiers introduce contravariance by allowing a type to be treated as a supertype, limiting how types can safely interact within generics.

The distinction between universal and existential quantifiers in type theory fundamentally impacts how we design and implement generics in Scala, emphasizing safe type interactions.

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