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Motivation for Language Specification

Motivation for Language Specification. Languages. Natural Languages. Artificial Languages. (descriptive). (prescriptive). Logic Languages. Programming Languages. Aspects. Syntax. Semantics. Pragmatics. Motivation for Specifying Natural Languages.

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Motivation for Language Specification

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  1. Motivation for Language Specification L2SpecIntro

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

  3. Motivation for SpecifyingNatural Languages A lexicographer's business is solely to collect, arrange, and define the words that usage presents to his hands. He has no right to proscribe words; he is to present them as they are. -Noah Webster, lexicographer (1758-1843)  L2SpecIntro

  4. 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

  5. Semantics seeks profound definitions rather than difficult theorems. • Software Design • Use Divide and Conquer. Team effort common. • 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 • Scheme – Guy Steele, Python – Guido von Rossum • Scala – Martin Oderksy, Clojure– Rich Hickey L2SpecIntro

  6. Problems with Informal Specifications Some Historically Significant Examples of Ambiguity/Incompleteness L2SpecIntro

  7. 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

  8. 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

  9. Semantic Equivalence Example Are the following statement templates equivalent? while <cond> do <statement> vs 25: if <cond> then begin <statement>; goto 25 end; 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--); } } while (k1--); printf("\n"); do { j = 2; TAG: if ( j > 0 ) { printf("\t j = %d \n", j--); goto TAG; } } while (k2--); } L2SpecIntro

  11. #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

  12. Cause of ‘The 1990 AT&T Long Distance Network Collapse’ • The defect was a C program that featured a break statement located within an if-then-else statement, that was nested within a switch statement. • Instead of the control breaking out of the else clause, it also broke out of the surrounding switch. L2SpecIntro

  13. 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

  14. 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

  15. 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

  16. 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

  17. CS7100 Agenda Language Syntax(BNF) Semantics Pragmatics Data Control Abstract Data Types Denotational AxiomaticOperational (interpreter-based) Attribute Grammar Framework L2SpecIntro

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