Avoiding The Need For Covariant Argument Types
The need for covariant argument types arises from strategies in object oriented languages for context-sensitive selection of the code used in method calls. This does not include the first parameter of a method call, which is the object itself, which is not contravariant.
Castagna showed that types used for runtime selection of the method are covariant, while types not used for runtime selection of the method are contravariant. In Castagna's work, examples which would suggest the usage of covariance for parameter types are treated with the usage of multiple dispatch, i.e. overriding where the right method is selected also based on the type of some arguments; applying the rule, covariance is allowed for those argument types. However, this solution cannot be applied to most programming languages, since they do not support multiple dispatch.
Note that for (static) overload resolution, the opposite rule applies: types used for compile-time method selection (i.e. parameter types) are contravariant; types not used to select the method are covariant.
These terms are also used in the context of modern programming languages that offer other functors to create new types with type variables, e.g., generic programming or parametric polymorphism, and exception handling where method definitions are enriched with annotations that indicate possible failures.
Read more about this topic: Covariance And Contravariance (computer Science)
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