Covariance and contravariance (computer science): Difference between revisions

Revision as of 19:24, 22 June 2023

Revision as of 20:55, 11 July 2023

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On the other hand, "function from Animal to String" is a subtype of "function from Cat to String" because the function type constructor is '''contravariant''' in the [[parameter (computer science)|parameter]] type. Here, the subtyping relation of the simple types is reversed for the complex types.

In other words, covariance is the quality of being different by being more specific ({{C sharp|Cat}} is covariant to {{C sharp|Animal}}) while contravariance is the quality of being different by being more general ({{C sharp|Animal}} is contravariant to {{C sharp|Cat}}).

A programming language designer will consider variance when devising [[typing rule]]s for language features such as [[Array (data type)|array]]s, [[Inheritance (object-oriented programming)|inheritance]], and [[generic datatype]]s. By making type constructors covariant or contravariant instead of '''invariant''', more programs will be accepted as well-typed. On the other hand, programmers often find contravariance unintuitive, and accurately tracking variance to avoid [[runtime errors|runtime type errors]] can lead to complex typing rules.