Type variance

Common card · No. 441892

In computer programming, type variance is the relationship between subtypes of a composite type (e.g. List[Int]) and the subtypes of its components (e.g. Int). A language's chosen variance determines the relationship between, for example, a list of Cats and a list of Animals, or a function returning Cat and a function returning Animal. If the type Cat is a subtype of Animal, then an expression of type Cat should be substitutable wherever an expression of type Animal is used. Depending on the variance of the type constructor, the subtyping relation of the simple types may be either preserved, reversed, or ignored for the respective complex types. In many programming languages, for example, "list of Cat" will be a subtype of "list of Animal", because the list type constructor is covariant. This means that the subtyping relation of the simple types is preserved for the complex types. On the other hand, "function from Animal to String" is a subtype of "function from Cat to String" because...

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