otypes

Otypes are a formal concept used in some type-theory frameworks to describe object types defined by explicit interfaces. An otype specifies a contract that objects must satisfy, consisting of a set of named properties (fields) and a set of operations (methods) with declared input and output types. Objects typed as an otype are expected to provide the declared structure and behavior, enabling static reasoning about program correctness and data interchange.

Structurally, an otype combines data shape and behavior. The properties describe the visible state an object must expose, while the methods define the operations available on that state. Subtyping among otypes is typically defined by structural refinement: a subtype must provide all properties of its supertype, possibly with additional properties or stricter constraints. This allows a single otype to be used in contexts requiring the broader interface, while more specialized otypes can be substituted when extra guarantees are available.

Typing discipline for otypes can be static, dynamic, or hybrid. In static settings, a compiler checks that expressions conform to the otypes’ interfaces. In dynamic settings, runtime checks may ensure that objects implement the required properties before operations are invoked. Otypes are often contrasted with nominal-type systems, since their emphasis is on structural conformance rather than declared inheritance, though they can be integrated with nominal metadata where needed.

Examples illustrate the concept: an otype Point2D might require x and y as numbers and a method distance(): Float; a ColoredPoint otype could extend Point2D by adding color: String. Otypes are discussed in theoretical literature and used in domain-specific languages for data modeling and interoperation.

See also: type system, structural typing, nominal typing, subtyping, object-oriented programming.