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Precondition
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In computer programming, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code or before an operation in a formal specification.
If a precondition is violated, the effect of the section of code becomes undefined and thus may or may not carry out its intended work. Preconditions that are missing, insufficient, or not formally proved (or have an incorrect attempted proof), or are not checked statically or dynamically, can give rise to Security problems, particularly in unsafe languages that are not strongly typed.
Often, preconditions are simply included in the documentation of the affected section of code. Preconditions are sometimes tested using guards or assertions within the code itself, and some languages have specific syntactic constructions for doing so.
Example
[edit]The factorial function is only defined where its parameter is an integer greater than or equal to zero. So an implementation of the factorial function would have a precondition that its parameter be an integer and that the parameter be greater than or equal to zero. Alternatively the type system of the language may be used to specify that the parameter of the factorial function is a natural number (unsigned integer), which can be formally verified automatically by a compiler's type checker.
In addition where numeric types have a limited range (as they do in most programming languages) the precondition must also specify the maximum value that the parameter may have if overflow is not to occur. (e.g. if an implementation of factorial returns the result in a 64-bit unsigned integer then the parameter must be less than 21 because factorial(21) is larger than the maximum unsigned integer that can be stored in 64 bits). Where the language supports range sub-types (e.g. Ada) such constraints can be automatically verified by the type system. More complex constraints can be formally verified interactively with a proof assistant.
In object-oriented programming
[edit]Preconditions in object-oriented software development are an essential part of design by contract. Design by contract also includes notions of postcondition and class invariant.
The precondition for any routine defines any constraints on object state which are necessary for successful execution. From the program developer's viewpoint, this constitutes the routine caller's portion of the contract. The caller then is obliged to ensure that the precondition holds prior to calling the routine. The reward for the caller's effort is expressed in the called routine's postcondition.[1]
Eiffel example
[edit]The routine in the following example written in Eiffel takes as an argument an integer which must be a valid value for an hour of the day, i. e., 0 through 23, inclusively. The precondition follows the keyword require. It specifies that the argument must be greater than or equal to zero and less than 24. The tag valid_argument describes this precondition clause and serves to identify it in case of a runtime precondition violation.
set_hour (a_hour: INTEGER)
-- Set `hour' to `a_hour'
require
valid_argument: 0 <= a_hour and a_hour < 24
do
hour := a_hour
ensure
hour_set: hour = a_hour
end
Preconditions and inheritance
[edit]In the presence of inheritance, the routines inherited by descendant classes (subclasses) do so with their preconditions in force. This means that any implementations or redefinitions of inherited routines also have to be written to comply with their inherited contract. Preconditions can be modified in redefined routines, but they may only be weakened.[2] That is, the redefined routine may lessen the obligation of the client, but not increase it.
See also
[edit]- Design by contract
- Guard (computer science)
- Postcondition
- Hoare logic
- Invariants maintained by conditions
- Database trigger
References
[edit]- ^ Meyer, Bertrand, Object-Oriented Software Construction, second edition, Prentice Hall, 1997, p. 342.
- ^ Meyer, 1997, pp. 570–573.
Precondition
View on Grokipediafrom Grokipedia
A precondition is a condition or predicate that must be satisfied prior to the execution or application of a process, operation, or action to ensure its validity or correctness. This concept appears across various fields, including logic, mathematics, everyday language, and law, where it denotes prerequisites for an event or procedure. In computer programming, a precondition specifically refers to a logical condition that must hold true immediately before executing a segment of code, function, or procedure to ensure correct operation and avoid undefined behavior.[1] It represents assumptions made by the implementer about the program's state or inputs at invocation, placing responsibility on the caller to meet these requirements.[2] Preconditions are often expressed as assertions or formal specifications and paired with postconditions, describing the expected state after execution, to support program verification and debugging.[3]
The formal use of preconditions originated in axiomatic semantics for proving correctness, notably through Hoare logic, developed by C. A. R. Hoare in his 1969 paper "An Axiomatic Basis for Computer Programming."[4] In Hoare triples {P} C {Q}, P is the precondition, C the code, and Q the postcondition; the logic deduces that if P holds before C, then Q holds after, enabling mathematical proofs of program properties.[5] This influenced developments like weakest precondition semantics by Edsger Dijkstra, computing minimal conditions for postconditions.[6]
Preconditions became prominent in software engineering via design by contract (DbC), introduced by Bertrand Meyer in the 1980s and core to the Eiffel programming language.[7] In DbC, preconditions define client obligations to the routine, with postconditions and invariants as supplier guarantees, fostering modular, reliable software through enforceable specifications. Modern languages like Java (with assertions) and Ada (runtime checks) incorporate these for error detection and robustness.[8] Analogous ideas extend to formal methods such as model checking and mathematical preconditioning for iterative solvers in linear systems.[9]
Here, the precondition
This example, adapted from standard academic demonstrations of preconditions, explicitly checks the condition and signals an error if violated, preventing the function from attempting an invalid operation.[43]
The precondition here ensures domain validity by verifying that the input is non-negative, thereby avoiding invalid computations such as attempting to take the square root of a negative number, which could lead to mathematical errors or undefined behavior in the underlying implementation.[43] By documenting and enforcing this condition, the function communicates clear expectations to callers, aligning with the Design by Contract paradigm where routines specify their required inputs.[43]
A common pitfall in implementing preconditions is forgetting to document them in comments or specifications, or neglecting to include runtime checks, which can result in silent failures or runtime errors when invalid inputs are provided, leading to unpredictable program behavior such as crashes or data corruption.[43]
Preconditions align with formal verification standards like DO-178C for safety-critical software in avionics, published in 2011, where such checks support robust error handling and verification objectives through supplements like DO-333 on formal methods.
General Definition
In Logic and Mathematics
In logic and mathematics, a precondition is defined as a condition or set of conditions that must hold true prior to performing an operation, making an inference, or applying a theorem, ensuring the validity of the subsequent result.[10] This concept underpins formal reasoning by specifying the assumptions necessary for a conclusion to follow necessarily. In logical statements, the precondition often appears as the antecedent in an implication, where its truth guarantees the truth of the consequent under the rules of inference. The historical origins of preconditions in logic can be traced to Aristotle's development of syllogistic reasoning in his Prior Analytics around 350 BCE, where premises function as preconditions that, when supposed to be true, yield a necessary conclusion distinct from those premises.[11] Aristotle described a syllogism as "a discourse in which, certain things being stated, something other than what is stated follows of necessity from their being so," emphasizing the premises' role in establishing the logical necessity of the outcome.[11] This foundational approach influenced Western logic for centuries, though it was limited to categorical statements without explicit quantification. Modern formalizations advanced in the late 19th and early 20th centuries through Gottlob Frege's Begriffsschrift (1879), which introduced a symbolic notation for conditional judgments, treating the antecedent as a precondition whose affirmation leads to the consequent via detachment rules. Bertrand Russell, building on Frege's work in Principia Mathematica (1910–1913) with Alfred North Whitehead, further refined logical implications within a ramified type theory, where preconditions in implications ensure the derivation of mathematical truths from logical axioms alone.[12] In mathematical contexts, preconditions appear in theorem statements of the form "If (precondition), then ," where must be satisfied for to hold, as formalized by the logical implication .[10] For instance, in calculus, continuity at a point serves as a precondition for differentiability: if a function is differentiable at , then is continuous at . This theorem highlights how failing the precondition (discontinuity) precludes the operation (differentiation), underscoring the hierarchical dependencies in mathematical proofs.In Everyday and Legal Contexts
In everyday language, a precondition refers to a requirement or assumption that must be fulfilled before an action or event can proceed.[13] For instance, possessing a valid passport serves as a precondition for international travel, ensuring compliance with border regulations before departure.[14] This concept underscores the necessity of preparatory conditions to enable subsequent outcomes, much like prerequisites in daily planning.[15] In legal contexts, preconditions often manifest as conditions precedent in contracts, which are events or stipulations that must occur before contractual obligations become enforceable.[16] For example, contingent clauses in real estate agreements may require a home inspection or financing approval as preconditions to finalizing the sale, protecting parties from proceeding without essential safeguards.[17] These mechanisms ensure that agreements are only binding once specified criteria are met, mitigating risks associated with uncertainty.[18] Within tort law, foreseeability acts as a critical precondition for establishing liability in negligence cases, determining whether a defendant could reasonably anticipate the harm caused by their actions.[19] Courts assess if the injury was a foreseeable consequence, thereby limiting liability to situations where risks were predictable and preventable.[20] This principle balances accountability with practical boundaries, preventing indefinite extension of responsibility.[21] Psychologically, cognitive preconditions such as prior knowledge play a foundational role in effective learning, influencing how individuals process and integrate new information.[22] Research indicates that learners with relevant background knowledge experience reduced cognitive load, enabling deeper engagement and better retention during instructional activities.[23] Without these preconditions, comprehension can be hindered, as new concepts build upon existing mental frameworks.[24] In behavioral economics, Daniel Kahneman's work on bounded rationality identifies key preconditions for rational decision-making, such as access to complete information and sufficient cognitive resources, which are often absent in real-world scenarios.[25] His prospect theory demonstrates that deviations from classical rationality arise due to heuristics and framing effects, challenging assumptions of utility maximization under uncertainty.[26] These insights reveal how intuitive judgments (System 1 thinking) frequently override deliberate analysis (System 2), leading to systematic biases in choices.[27] In international law, the Vienna Convention on the Law of Treaties (1969) specifies preconditions for treaty validity, requiring free and informed consent without vitiation by error, fraud, corruption, coercion, or conflict with jus cogens norms.[28] Articles 46–53 outline grounds for invalidity, such as coercion of representatives (Article 51) or treaties procured by force (Article 52), ensuring that only agreements meeting these standards are legally binding.[28] This framework upholds the integrity of state interactions by mandating procedural and substantive safeguards.[29]In Software Engineering
Design by Contract Paradigm
The Design by Contract (DbC) paradigm, introduced by Bertrand Meyer in the 1980s, treats software modules as formal contracts between suppliers (the implementers) and clients (the users), where each routine specifies mutual obligations through preconditions, postconditions, and class invariants to ensure reliable behavior.[30] This approach shifts software development from ad hoc coding to a disciplined process akin to legal agreements, emphasizing verifiable specifications over implicit assumptions.[30] Preconditions form a foundational element of DbC, defining the environmental conditions that the client must establish before calling a routine, such as ensuring an array index falls within bounds or a data structure is not empty.[30] By explicitly stating these caller responsibilities, preconditions enable the supplier to assume their validity, simplifying routine logic and focusing implementation efforts on delivering the postconditions—outcomes guaranteed if preconditions hold.[30] For instance, in a stack operation like popping an element, a precondition might require the stack to be non-empty, offloading input validation to the client and preventing unnecessary error-prone checks within the routine.[30] Implementing DbC yields significant benefits, including enhanced software reliability through clear delineation of responsibilities, which reduces integration errors and supports modular verification of components in isolation.[30] It also streamlines debugging by leveraging assertions to monitor contract adherence during execution, allowing developers to pinpoint violations systematically rather than tracing vague failures.[30] However, the paradigm incurs drawbacks, such as potential runtime overhead from evaluating assertions in production environments and the risk of overly restrictive preconditions that complicate client usage if not carefully designed.[31] When a precondition is violated, DbC prescribes handling it as a client fault by raising an exception, thereby halting execution and alerting the developer to correct the calling code, without obligating the supplier to provide fallback behavior or recovery mechanisms.[30] This strict enforcement promotes accountability across modules and aligns with the paradigm's goal of preventing subtle bugs from propagating through the system.[30]Assertion Mechanisms
Assertion mechanisms provide the technical means to enforce and verify preconditions in software, ensuring that functions or methods are invoked only under valid conditions. These mechanisms typically involve assertions, which are boolean expressions evaluated to confirm expected states; if false, they signal a violation, often halting execution or logging an error. In languages like C++ and Java, runtime assertions are implemented via standard libraries, such as theassert macro in C++'s <cassert> header or Java's assert keyword, allowing developers to embed precondition checks directly in code, e.g., assert(input > 0) before processing positive values.[32] These checks serve as lightweight runtime validations tied to the design by contract paradigm, where preconditions define contractual obligations for callers.[33]
Assertion types divide into compile-time and runtime variants, with broader categories encompassing static analysis tools versus dynamic testing approaches. Compile-time assertions, such as those using C++ template metaprogramming with static_assert (introduced in C++11), detect precondition violations before execution by leveraging the compiler's type system. Static analysis tools, including model checking techniques, formally verify preconditions across all possible program paths without running the code; for instance, tools like SPARK for Ada or general model checkers like CBMC for C analyze specifications to prove precondition adherence, reducing runtime overhead but requiring formal annotations. In contrast, dynamic testing relies on runtime evaluation during execution, often through unit tests that incorporate precondition stubs—mock setups simulating invalid inputs to trigger and observe failures. Frameworks like JUnit, first released in 1998, integrate assertions into test suites with methods such as assertTrue for precondition validation in Java, enabling automated regression testing.[34][35] Similarly, Pytest, originating in 2004 as part of the PyPy project's py library, enhances Python's built-in assert by providing rich introspection and fixtures for precondition-heavy tests, streamlining dynamic verification in modern development workflows.[36][33][37]
The evolution of assertion mechanisms traces back to early debugging aids in the 1970s, when runtime checkers emerged as preprocessors for languages like FORTRAN to insert precondition validations during development. By the 1980s and 1990s, assertions became standardized in languages, evolving from ad-hoc tools to integral components of testing ecosystems, as seen in the widespread adoption of JUnit and its influence on subsequent frameworks like Pytest. This progression reflects a shift from manual debugging to automated, scalable verification, with modern tools balancing performance and coverage through hybrid static-dynamic approaches.[33]
When preconditions fail, assertion mechanisms trigger specific failure modes to isolate issues, such as aborting execution in C++ via assert or throwing an AssertionError in Java, which aids debugging by pinpointing invalid calls. Defensive programming strategies mitigate these failures gracefully, emphasizing input sanitization and error recovery over abrupt termination; for example, instead of a fatal assert, code might validate parameters and return error codes or default values, ensuring system robustness in production environments where caller errors are anticipated. This approach contrasts with strict enforcement, prioritizing availability while logging violations for later analysis.[38]
In Object-Oriented Programming
Implementation in Eiffel
In the Eiffel programming language, preconditions are specified using therequire keyword within a routine declaration, allowing developers to define boolean expressions that must hold true before the routine executes.[39] These clauses can include optional tags for clarity, such as require non_negative: argument >= 0, ensuring that the specified conditions on inputs or object states are met.[39] This syntax integrates seamlessly with Eiffel's Design by Contract (DbC) methodology, promoting reliable software by explicitly documenting and verifying caller obligations.[40]
A practical example illustrates this in a simple division routine to avoid division by zero. Consider the following Eiffel code for a feature divide in a class:
divide (x, y: REAL): REAL
require
denominator_non_zero: y /= 0.0
do
Result := x / y
[ensure](/page/Ensure)
correct_result: Result * y = x
end
divide (x, y: REAL): REAL
require
denominator_non_zero: y /= 0.0
do
Result := x / y
[ensure](/page/Ensure)
correct_result: Result * y = x
end
denominator_non_zero: y /= 0.0 mandates that the divisor y must not be zero, preventing runtime errors and clarifying the routine's usage contract.[39] If violated, it signals a client-side error rather than a routine implementation fault.
Enforcement of preconditions in Eiffel combines static and dynamic mechanisms. The compiler performs static analysis for certain verifiable conditions, such as those related to void safety—introduced in the ECMA-367 standard in 2006—which guarantees that no void (null) calls occur by rejecting non-conforming code at compile time.[41] For dynamic preconditions that cannot be fully resolved statically, runtime checking is enabled through compilation options (e.g., asserting require clauses), raising a PRECONDITION_VIOLATION exception if falsified, thus facilitating debugging without embedding checks in the routine body.[39]
This approach to preconditions originated in Eiffel's design in 1985 by Bertrand Meyer, who integrated DbC natively to enforce software correctness through formal contracts rather than ad hoc testing.[42]
Role in Inheritance and Polymorphism
In the Design by Contract (DbC) paradigm, preconditions play a critical role in inheritance hierarchies by ensuring that subclasses maintain compatibility with parent classes, thereby upholding the Liskov Substitution Principle (LSP). Specifically, when overriding a method, a subclass must not strengthen the parent's precondition—meaning it cannot impose additional or stricter requirements on inputs—but may weaken it to allow a broader set of valid calls, preserving the substitutability of objects without altering the expected behavior for clients using the parent type. This rule prevents violations where a polymorphic call, expecting the parent's contract, fails due to unexpected constraints in the subclass, thus safeguarding the reliability of inheritance-based designs. This interaction is formalized through the concept of behavioral subtyping, where a type S is a subtype of T if every method in S satisfies T's behavioral expectations: the precondition of S's method must be implied by T's precondition (weaker or equal), and S's postcondition must imply T's postcondition (stronger or equal). Behavioral subtyping extends structural subtyping by focusing on observable behavior rather than just type compatibility, ensuring that polymorphic invocations respect all contractual obligations across the hierarchy. For instance, consider a parent classShape with a draw() method preconditioned on a valid canvas being provided; a subclass Circle overriding draw() might relax this to accept a null canvas by internally initializing one, allowing more flexible usage without breaking clients that rely on the parent's stricter guarantee.
Challenges arise from the contravariant nature of preconditions in subtyping, where weakening them in subclasses can lead to subtle design errors, such as unintended side effects or contract violations during runtime polymorphism if not carefully managed. For example, over-relaxing a precondition might expose internal state to invalid inputs that the parent assumed were filtered, potentially causing cascading failures in inherited code. To address these, tools like AutoProof, developed in the 2010s, automate verification of DbC properties in object-oriented programs, checking inheritance rules and generating proofs to detect contravariance mismatches early in development. Such tools enforce behavioral subtyping by translating contracts into logical assertions, ensuring polymorphic calls adhere to preconditions without manual intervention.
Practical Examples
Basic Code Illustration
A simple illustration of a precondition can be seen in a function designed to compute the square root of a non-negative number, ensuring the input falls within the valid domain before proceeding with the computation. In pseudocode, this might appear as follows:function sqrt_positive(x):
// Precondition: x >= 0
if x < 0:
raise Error("Input must be non-negative")
return sqrt(x)
function sqrt_positive(x):
// Precondition: x >= 0
if x < 0:
raise Error("Input must be non-negative")
return sqrt(x)