Declarefun

Declarefun is a programming concept and tooling idea that refers to a mechanism for declaring function interfaces without providing an implementation. A declarefun declaration specifies the function name, parameter types, return type, and optional contractual constraints, serving as a contract between a caller and an implementation.

Origin and scope: The concept emerged from discussions of interface description and contract-based programming. It appears as both a theoretical construct in language design and as a practical feature in experimental languages and libraries. In practice, declarefun declarations are used to define public APIs, generate bindings, or drive verification tools, while actual code provides the corresponding implementation elsewhere.

Usage and syntax: A declarefun line defines the signature and, optionally, preconditions and postconditions. Example: declarefun max(a: Int, b: Int) -> Int requires a >= 0 and b >= 0 ensures result >= max(a, b). The body is omitted in the declaration; a separate implementation must satisfy the contract. Some ecosystems use declarefun as a library construct rather than a language keyword, integrating with build-time tooling for checks and documentation.

Relation and limitations: Declarefun interacts with concepts such as contract programming, interface description languages, and API abstractions. It can improve modularity, tooling support, and cross-language interoperation, but relies on strong tooling to verify correctness and to resolve implementations. It is not universally supported and is often optional or experimental in most mainstream languages.