1. Motivation for Language Specification
cs784 (Prasad)
L2SpecIntro

2. Languages Natural Languages Artificial Languages Logic Languages
(descriptive)
Logic Languages
Programming Languages
(prescriptive)
Aspects
Syntax
Semantics
Pragmatics
cs784 (Prasad)
L2SpecIntro

3. Motivation for Specifying Semantics of Programming Languages
The language of mathematics is precise, well-understood, and standard.
In contrast, the PL notations are diverse and similar looking syntax have been given different meaning. So it is important to specify unambiguously what a system of notation stands for.
cs784 (Prasad)
L2SpecIntro

4. Semantics seeks profound definitions rather than difficult theorems.
Software Design
Use Divide and Conquer; Team effort.
Language Design
Unify and study interactions among language features. Individual effort common.
Pascal, Modula-2, Oberon - Niklaus Wirth
C - Dennis Ritchie
C++ - Bjarne Stroustrup
Java - James Gosling
C# - Anders Hejlsberg
cs784 (Prasad)
L2SpecIntro

5. Problems with Informal Specifications
Some Historically Significant Examples of Ambiguity/Incompleteness
cs784 (Prasad)
L2SpecIntro

6. Algol Example
Algol-60 report is ambiguous since it does not specify a fixed-order of evaluation for sub-expressions, or rule out the possibility of side-effects in programs.
If function f has side-effect on variable x then it is possible to have
x + f(x) =/= x + f(x)
f(x) + f(x) =/= 2 * f(x)
f(x) / f(x) =/= 1
Loss of Referential Transparency
cs784 (Prasad)
L2SpecIntro

7. Pascal Example Type Equivalence Structural Equivalence: {A,B,C,D,E}
type T = array [1..10] of integer;
var A,B : array [1..10] of integer;
C: array [1..10] of integer;
D: T ; E : T;
Structural Equivalence: {A,B,C,D,E}
Name Chain Equivalence: {A,B},{C},{D,E}
Name Equivalence: {A},{B},{C},{D,E}
cs784 (Prasad)
L2SpecIntro

8. Example Are the following statements equivalent?
while <cond> do <statement> vs
25: if <cond> then
begin <statement>;
goto 25
end;
These statements are equivalent in Pascal, Ada, etc. but not in C/C++/Java.
cs784 (Prasad)
L2SpecIntro

9. printf("\t i = %d \n", i--); }
#include <stdio.h>
main() {
int i, j, k1 = 2, k2 = 2;
do { i = 2;
while ( i > 0 ) {
printf("\t i = %d \n", i--); }
} while (k1--); printf("\n");
do { j = 2;
TAG:
if ( j > 0 ) {
printf("\t j = %d \n", j--);
goto TAG;
} while (k2--); /*
tcsh> gcc -ldl eq1.c
tcsh> a.out
i = 2
i = 1
j = 2
j = 1 */
cs784 (Prasad)
L2SpecIntro

10. printf("\t i = %d \n", i--); break; }
#include <stdio.h>
main() {
int i, j, k1 = 2, k2 = 2;
do { i = 2;
while ( i > 0 ) {
printf("\t i = %d \n", i--);
break; }
} while (k1--); printf("\n");
do { j = 2;
TAG:
if ( j > 0 ) {
printf("\t j = %d \n", j--);
goto TAG;
} while (k2--); /*
tcsh> gcc -ldl eq1.c
tcsh> a.out
i = 2
j = 2 */
cs784 (Prasad)
L2SpecIntro

11. Approaches to Formal Semantics
Operational : How a program executes?
Specifies abstract interpreter to carry-out the meaning of the programs.
Denotational : What a program computes?
Maps a program to a mathematical function from its inputs to its outputs.
Axiomatic : For reasoning about programs
Specifies properties of language constructs through pre-post conditions.
Abstraction level: OS < DS < AS
cs784 (Prasad)
L2SpecIntro

12. Interpreter vs Compiler
Interpreter evaluates the meaning of a program.
Compiler transforms a program in one language into an equivalent program in another language. That is, it preserves meaning.
Semantics of a language is independent of its machine implementation.
cs784 (Prasad)
L2SpecIntro

13. Why is formal semantics not widely used?
It is relatively complex and not yet cost effective for everyday (and everybody’s) use.
Semantics is general. In particular, it must consider all possible situations (including the boundary cases).
However, most programmers want to know:
What is the output for the particular inputs?
cs784 (Prasad)
L2SpecIntro

14. Who needs semantics?
Those who write (meta-)programs that must work for all programs.
Designers of
program transformation tools.
compilers and interpreters.
program analyzers.
software engineering tools.
critical software.
cs784 (Prasad)
L2SpecIntro

15. Syntax(BNF) Semantics Pragmatics
CS784 Agenda
Language
Syntax(BNF) Semantics Pragmatics
Data Control
Abstract Data Types
Denotational Axiomatic Operational
(interpreter-based)
Attribute Grammar Framework
cs784(Prasad)
L2SpecIntro