Motivation for Language Specification

1. Motivation for Language Specification L2SpecIntro

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

5. Problems with Informal Specifications Some Historically Significant Examples of Ambiguity/Incompleteness 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 fhas side-effect on variable xthen 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 L2SpecIntro

7. Pascal Example Type Equivalence 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} L2SpecIntro

8. Example Are the following statements equivalent? while <cond> do <statement> vs 25: if <cond> then begin <statement>; goto 25 end; L2SpecIntro

9. #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--); } L2SpecIntro

10. #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--); break; goto TAG; } } while (k2--); } 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 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. 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? 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. L2SpecIntro

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