Covariance and contravariance (computer science)

Many programming language type systems support subtyping. For instance, if the type <span class="n">Cat</span> is a subtype of <span class="n">Animal</span>, then an expression of type <span class="n">Cat</span> should be substitutable wherever an expression of type <span class="n">Animal</span> is used. 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 type. Here, the subtyping relation of the simple types is reversed for the complex types.