selectivetype

Selectivetype is a term used in discussions of typing and type systems to describe a type that is selected from a set based on a selector value or runtime predicate. It is not a standard feature in mainstream languages, but appears in theoretical descriptions and language design proposals as a way to express finer-grained type information without duplicating code.

Definition: A selectivetype represents a contract that a value will conform to exactly one member of a closed family of types, selected by a discriminator or condition. Once chosen, operations on the value are validated against that specific type, enabling more precise method availability and safety guarantees on that code path.

Mechanism: Selection can occur through pattern matching, type guards, or dependent types that refine the static type based on runtime information. In practice, a selectivetype enables the compiler or analyzer to narrow a variable from a broader union to a particular variant along each control-flow path, guiding type checking and error prevention.

Relation to existing concepts: Selectivetype is related to discriminated unions, sum types, and refinement types. It can be viewed as a formalization of the intuition behind narrowing via guards, or as a programmable discriminator that determines the active variant of a type set. This bridges ideas from dynamic type checks to static type inference.

Usage and considerations: Proponents argue that selectivetype improves safety and expressiveness in type-rich languages by aligning types with control-flow. Critics point to potential complexity in type inference and possible ambiguity in how discrimination is specified. As a concept, it serves as a lens for thinking about how types can adapt to program behavior.