Covariance vs. Contravariance: How Do These Concepts Differ in Programming?

Covariance and Contravariance: A Clear Distinction in Programming

Covariance and contravariance are fundamental concepts in programming that define how type relationships influence function mappings and data structure input/output types. Essentially, covariance preserves assignability direction, while contravariance reverses it.

Covariance: Maintaining Assignability

Let's illustrate with type sets:

<code>{ Animal, Tiger, Fruit, Banana }
{ IEnumerable<Animal>, IEnumerable<Tiger>, IEnumerable<Fruit>, IEnumerable<Banana> }</code>

The mapping T → IEnumerable maintains assignability. If Tiger is a subtype of Animal, then IEnumerable<Tiger> is also a subtype of IEnumerable<Animal>. This is common in container types, where subtyping applies to the contained elements.

Contravariance: Inverting Assignability

Now, consider these type sets:

<code>{ IComparable<Tiger>, IComparable<Animal>, IComparable<Fruit>, IComparable<Banana> }</code>

The mapping T → IComparable inverts assignability. If Animal is a supertype of Tiger, then IComparable<Animal> is a subtype of IComparable<Tiger>. This is frequently observed in functional types, where contravariant types manage input parameters of specific or related types.

Key Differences Summarized

Covariance upholds assignability direction; a subtype value can be assigned to a supertype value in both function input and output. In contrast, contravariance inverts assignability, enabling a supertype value assignment to a subtype value in the input parameter, but not in the output.

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