Within the type system of a programming language, covariance and contravariance refers to the ordering of types from narrower to wider and their interchangeability or equivalence in certain situations (such as parameters, generics, and return types)

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Variance is a topic that comes up fairly often in type systems and can be a bit confusing the first time you hear it. Let’s walk through each form of variance.

Variance refers to how subtyping between composite types (e.g. list of Cats versus list of Mammals) relates to subtyping between their components (e.g. Cats and Mammals).