Preconditionspostconditions

Preconditionspostconditions is a term used to describe the combined specification of a function's preconditions and postconditions in software design by contract. It codifies the requirements that must hold before a computation begins and the guarantees that hold after it completes, assuming the preconditions were satisfied.

Preconditions are predicates that define the allowed inputs or state required to safely execute a function. If a precondition is not met, the function is not obliged to perform correctly, and the behavior may be undefined, raise an error, or trigger safety checks. They serve as the contract’s left side, clarifying what callers must ensure before calling the function.

Postconditions state the conditions that will be true after execution, provided the preconditions held and the function terminates normally. They express the observable effects of the function, such as return values, changes to object state, or resource usage. In formal terms, they are the postconditions of a Hoare triple {P} C {Q}, where P is the precondition, C is the command, and Q is the postcondition.

Usage and language support: The concept is central to design by contract and formal verification. Some languages provide native support for contracts or annotations, such as Eiffel, Spec#, and Dafny, while others use libraries or static analyzers to check preconditions and postconditions at runtime or compile time.

Advantages and pitfalls: Clearer interfaces, easier debugging, and potential automated verification are common benefits. Pitfalls include overly strong or ambiguous contracts, performance costs from checks, and the risk that postconditions reveal too much implementation detail or hamper refactoring.

Example: A function divide(a, b) with precondition b != 0 and postcondition result == a / b. If b is zero, the precondition fails and the postcondition need not hold.