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Describing Syntax and Semantics

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1 Describing Syntax and Semantics
Lecture 17 Describing Syntax and Semantics Formal Methods in Software Engineering

2 Semantics How do we describe the “meaning” of a program?
Dynamic semantics or semantics is concerned with accurately describing the execution behaviour of a language Why do we care? English descriptions are often incomplete and ambiguous Compiler writers must implement the language description accurately Programmers want the same behaviour on different platforms There is no single widely acceptable notation or formalism for describing semantics Entire books have been dedicated to various semantic notations! Formal Methods in Software Engineering

3 Operational Semantics
Describe the meaning of a program by executing its statements on a machine, either simulated or actual. The change in the state of the machine (memory, registers, etc.) defines the meaning of the statement At the highest level, we’re interested in the final result (natural operational semantics) At the lowest level, look at a translated version to determine precise meaning of a single statement (structural operational semantics) Formal Methods in Software Engineering

4 Operational Semantics Example
C Statement for (expr1; expr2; expr3) { . . . } Operational Statements expr1; loop: if expr2 == 0 goto out . . . expr3; goto loop out: . . . Human reader is virtual computer, assumed to be able to correctly “execute” the instructions and recognize the effects. Note that language is intermediate level, not machine language. Formal Methods in Software Engineering

5 Operational Semantics (continued)
A better alternative: A complete computer simulation The process: Build a translator (translates source code to the machine code of an idealized computer) Build a simulator for the idealized computer Formal Methods in Software Engineering

6 Evaluation of Operational Semantics
Good if used informally (language manuals, etc.) Extremely complex if used formally (e.g., Vienna Definition Language), it was used for describing semantics of PL/I. Can lead to circularities, because statements of high-level language are described in statements of lower-level language These problems can be avoided with formalisms based on logic or mathematics Formal Methods in Software Engineering

7 Axiomatic Semantics Based on formal logic (predicate calculus)
Original purpose: formal program verification Correctness proofs specify constraints on program variables When proofs can be constructed, they show that a program performs the computation described by its specification Formal Methods in Software Engineering

8 Axiomatic Semantics (continued)
Logical expressions used in axiomatic semantics are called assertions. An assertion before a statement (a precondition) states the relationships and constraints among variables that are true at that point in execution An assertion following a statement is a postcondition A weakest precondition is the least restrictive precondition that will guarantee the postcondition Formal Methods in Software Engineering

9 Axiomatic Semantics Form
precondition postcondition Pre-, post form: {P} statement {Q} An example a = b + 1 {a > 1} One possible precondition: {b > 10} Weakest precondition: {b > 0} Formal Methods in Software Engineering

10 Program Proof Process The postcondition for the entire program is the desired result Work back through the program to the first statement. If the precondition on the first statement is the same as the program specification, the program is correct. An axiom is a logical statement that is assumed to be true. An inference rule is a method of inferring the truth of one assertion on the basis of the value of other assertions. Formal Methods in Software Engineering

11 Program Proof Process (cont)
To use axiomatic semantics with a given programming language (either for correctness proofs or for formal semantic specification), must have either an axiom or an inference rule for each kind of statement in the language The following rules assume that expressions do not have side effects Formal Methods in Software Engineering

12 Axiomatic Semantics: Assignment
An axiom for assignment statements (x = E): {Qx->E} x = E {Q} Example 1 a = b/2 – 1 {a < 10} means b/2-1 must be < 10, or b < 22 is precondition Example 2 x = x + y – 3 { x > 10} means x + y – 3 > 10, so y > 13 – x OK for variable to be on both sides Q is constraint on x replace x by E in Q Formal Methods in Software Engineering

13 Axiomatic Semantics - Consequence
{x > 3} x = x – 3 { x > 0} Using assignment axiom, x = x – 3 {x > 0} produces precondition of { x > 3 } which “proves” this statement What about {x > 5} x = x – 3 { x > 0 } ?? The Rule of Consequence: Formal Methods in Software Engineering

14 Axiomatic Semantics: Sequences
{P1} S1 {P2} {P2} S2 {P3} Formal Methods in Software Engineering

15 Axiomatic Semantics: Selection
if B then S1 else S2 Must be proven for both true and false conditions Example: if (x > 0) y = y-1 else y = y + 1 Assume Q is {y > 0} then P is {y > 1} else P is {y > -1} Since {y > 1} => {y > -1} use {y > 1} one P one Q Formal Methods in Software Engineering

16 Axiomatic Semantics: Pretest Loops
{P} while B do S end {Q} Number of iterations not always known. Use a loop invariant I and induction. Formal Methods in Software Engineering

17 Axiomatic Semantics: Loops
Characteristics of the loop invariant: I must meet the following conditions: P => I the loop invariant must be true initially {I} B {I} evaluation of the Boolean must not change the validity of I {I and B} S {I} -- I is not changed by executing the body of the loop (I and (not B)) => Q if I is true and B is false, is implied The loop terminates Formal Methods in Software Engineering

18 Axiomatic Semantics: Loop Example
assignment equality test while y <> x do y = y + 1 { y = x} Run through the loop a few times to find weakest precondition 1st: wp (y = y + 1, {y = x} } = {y + 1 = x or y = x – 1} 2nd: wp (y = y + 1, {y = x} } = {y + 1 = x or y = x – 2} 3rd: wp (y = y + 1, {y = x} } = {y + 1 = x or y = x – 3} So we see that {y < x} will suffice for 1 or more iterations. Combined with {y = x} for 0 iterations we have { y <= x } for loop invariant. I can also be used as the precondition. NOTE: text walks through four conditions for I Formal Methods in Software Engineering

19 Loop Invariant The loop invariant I is a weakened version of the loop postcondition, and it is also a precondition. I must be weak enough to be satisfied prior to the beginning of the loop, but when combined with the loop exit condition, it must be strong enough to force the truth of the postcondition Finding loop invariant can be difficult. If loop termination can be shown, axiomatic description is called total correctness. If other conditions can be met but termination is not guaranteed, called partial correctness. Formal Methods in Software Engineering

20 Evaluation of Axiomatic Semantics
Developing axioms or inference rules for all of the statements in a language is difficult It is a good tool for correctness proofs, and an excellent framework for reasoning about programs, but it is not as useful for language users and compiler writers Its usefulness in describing the meaning of a programming language is limited for language users or compiler writers Formal Methods in Software Engineering

21 Denotational Semantics
Based on recursive function theory The most abstract semantics description method Originally developed by Scott and Strachey (1970) Formal Methods in Software Engineering

22 Denotational Semantics (continued)
The process of building a denotational specification for a language: Define a mathematical object for each language entity Define a function that maps instances of the language entities onto instances of the corresponding mathematical objects The meaning of language constructs are defined by only the values of the program's variables Formal Methods in Software Engineering

23 Denotation Semantics vs Operational Semantics
In operational semantics, the state changes are defined by coded algorithms In denotational semantics, the state changes are defined by rigorous mathematical functions Formal Methods in Software Engineering

24 Evaluation of Denotational Semantics
Can be used to prove the correctness of programs Provides a rigorous way to think about programs Can be an aid to language design Has been used in compiler generation systems (but no useful compilers generated) Because of its complexity, they are of little use to language users Formal Methods in Software Engineering

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