Programming Language Descriptions

PL-Descriptions 2

What drives PL Development?

• Computers are “in charge” of extremely important issues• Execute a program literally.• Exercise no “judgment”.

• Program development• “works” : not too hard• “works correctly”: beyond the state of the art

• Errors are due to inadequate command over the • programming domain• programming language

• A PL design should• Make programs efficient• Make programmers more productive

• Help catch typical errors

• High level operations

• PL easy to understand

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PL-Descriptions 3

Division of PL Descriptions

• Syntax– Context Free Grammars– Context Sensitive Details

• Semantics– PL designer typically uses prose.– Others would have developed

• Operational Semantics• Axiomatic Semantics• Denotational

• Pragmatics– Various practical details– Prose

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PL-Descriptions 4

Description of Syntax: BNF

• Backus-Naur-Form– Context-Free only– LHS ::= seq of Terminals/Non-Terminals– ::= separates LHS from RHS– Repetition/Kleene Star { … }*

• Many variations of BNF

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PL-Descriptions 5

Description of Semantics

• What does a program “mean”?

• Mostly via carefully written prose.

• Semantics of a language is (ought to be) independent of its machine implementation.

• Subtleties abound

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PL-Descriptions 6

Side Effects

• If function f has side-effect on variable x then it is possible to have

x + f(x) =/= f(x) + x

f(x) + f(x) =/= 2 * f(x)

f(x) / f(x) =/= 1, even assuming f(x) != 0

• Loss of Referential Transparency

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PL-Descriptions 7

Subtle variations in semantics

• Are the following equivalent?• (Ignore PL specific syntactic details.)

• while B do S• S stands for a statement sequence

• 25: if B then begin S; goto 25 end

• Equivalent in Pascal, Ada, etc., but not in C/C++/Java.

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PL-Descriptions 8

Scope of break statement

#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--);}

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PL-Descriptions 9

Type Equivalence

type T = array [1..10] of integer;

var A,B : array [1..10] of integer;

C: array [1..10] of integer;

D, E: T;

Structural Equivalence: {A,B,C,D,E}

Name Chain Equivalence: {A,B},{C},{D,E}

Name Equivalence: {A},{B},{C},{D,E}

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PL-Descriptions 10

Interpreter vs Compiler

• Interpreter evaluates the representation of a program.

• Compiler transforms a program representation in one language into an “equivalent” program in another language.

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Why is formal semantics not widely used?

• Real PLs are too complex.

• 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).

• But steady progress is being made ...

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PL-Descriptions 12

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

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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.

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PL-Descriptions 14

Language

Syntax(BNF) Semantics Pragmatics

Data Control

Abstract Data Types

Denotational Axiomatic Operational

(interpreter-based)

Attribute Grammar Framework

CS784 Agenda

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