functionmust

Functionmust is a proposed concept in programming and formal methods describing a set of obligations that a function must satisfy. In this view, a function is annotated with functionmust clauses that specify required properties of its behavior, such as preconditions, postconditions, and invariants. Functionmust is not a formal standard, but is used in discussions of contract-based design to illustrate how function behavior can be specified and verified.

Semantically, a function with functionmust clauses is expected to be correct for all inputs that meet its preconditions, guarantee the stated postconditions on termination, and preserve invariants. The idea is closely related to design by contract, contracts, and program verification, but keeps the naming flexible for varied toolchains and languages.

Example: a functionmust for a simple increment operation might state: precondition x >= 0; postcondition result == x + 1; termination guaranteed. A contract checker or verifier can then verify that the implementation conforms to these claims.

Critics argue that functionmust can add annotation overhead and may signal requirements beyond what languages provide natively. Supporters say it improves clarity, enables automated testing and formal verification, and helps separate interface obligations from implementation details.

See also: design by contract, precondition, postcondition, invariant, contract programming, formal verification.