Contravariance
Contravariance is a type-system relationship in which subtyping between parameterized types reverses the subtyping relationship between their type arguments, typically for input positions.
Kotlin in Action, 2E
A contravariant type parameter is a generic type parameter that reverses the subtype direction of its type arguments and is restricted to in positions.
- How it is used
- Models consumer-style Kotlin APIs so handlers for supertypes can be supplied where handlers for subtypes are required.
- How deep it goes
- Defines reversed generic subtyping through Comparator and function examples, then applies it to star-projected validators and casts.
- Examples
- Uses fruit weight comparators, an Animal extension passed for cats, and a `KClass`-keyed `FieldValidator` registry.
- What you can do after
- Evaluate/understand when an `in`-typed consumer can safely replace a consumer of a narrower type and why star projections reject inputs.
Fabulous Adventures in Data Structures and Algorithms
Contravariance is a type-system relationship in which a more-derived type parameter can substitute for a less-derived type parameter in input positions.
- How it is used
- Explains why input-only generic interfaces support assignment conversions opposite to the underlying type relationship.
- How deep it goes
- Explains input positions, reversed assignments, and categorical morphisms, but treats category theory as only a surface-level introduction.
- Examples
- Uses Animal, Giraffe, and Tiger types with an IC<T> comparison interface and an IC<Animal> to IC<Giraffe> assignment.
- What you can do after
- Evaluate type assignments involving C# contravariant interfaces and explain their reversed compatibility using input positions.
Read how Fabulous Adventures in Data Structures and Algorithms explains contravariance →
Related concepts
- covariancecontrasts with · Covariance is a type-system property in which subtyping of a type argument induces the same-direction subtyping relationship for its parameterized type.
- in positionuses · An in position is a type-parameter usage position in which values of the parameterized type are accepted or consumed rather than produced.
- comparison functionused by · A comparison function or strategy compares two values and indicates their relative ordering, and sometimes whether they are equivalent.
- assignment compatibilityproduces · Assignment compatibility is a type-system relation determining whether a value or expression of one type can be assigned to an object of another type.
- membership operatorconfigured by · A membership operator is a language construct that tests whether a specified value or property is present in a supported collection, range, or object.
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