instancefor

Instancefor is a declarative construct used in some programming libraries to specify how to generate a concrete instance of a given type. It associates a type with a generator function or recipe that constructs values, typically for testing, mock data, or fixtures. The goal is to separate the data structure definition from the logic used to instantiate it in different contexts.

Usage and syntax commonly involve declaring an instancefor block or directive that binds a type to a generator. For example, in a hypothetical language you might declare:

instancefor User where

generator = User(id = autoId, name = autoName, email = autoEmail)

This defines how to create a User value automatically. Many implementations also support overrides or overrides blocks to customize specific fields without changing the default generator, such as:

instancefor User with overrides { name = "Alice" }

Common patterns and capabilities include default field values, nested generation for composite types, and integration with testing or fixture pipelines. Instancefor can be used to produce deterministic or randomized data, depending on the underlying generator, and may include constraints or filters to ensure generated values meet domain rules.

Relation to other concepts: instancefor is related to factory patterns, builders, and fixture libraries. It shares goals with property-based testing data generators by providing systematic ways to produce valid instances, but it typically focuses on the mechanics of instantiation rather than test case reduction or hypothesis coverage.

See also: factory, builder pattern, fixture, property-based testing, mock data generation.