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On Languages

History of Programming Languages

About CompilersSyntax and Semantics

Describing Syntax:BNF and EBNF

Syntax Diagrams

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Læringsmål• Kunne forklare de forskellige opgaver, som en

compiler udfører• Kunne læse og forstå syntaksbeskrivelser på

forskellig form, især BNF• Kunne anvende regulære udtryk.• Kunne anvende tilstandsmaskiner til at

genkende regulære udtryk.• Kende sammenhængen mellem regulære

udtryk og tilstandsmaskiner.

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Some importantProgramminglanguages andtheir relationships

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The Tasks of the Compiler

CompilerSource Program Object Code

Error messages

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The Structure of a Compiler

sourcescanner

tokensparser

syntactic structure

SemanticRoutines

intermediate code optimizer Code

generatorobject code

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Syntax and Semantics

• The syntax of a language:– Rules for forming valid (legal) constructions

(sentences) in the language.• The semantics of a language:

– The meaning that is put into valid constructions in the language.

So syntax is about form and semantics about meaning.

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Examples

”The man drinks the beer that is cold”Syntactically and semantically correct.

”The man drink the beer that are coldly.” Not syntactically correct (but semantics?)

”The beer drinks the man, who is cold”Syntactically correct, but semantics is rubbish.

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Natural Languages vs. Context Free Languages

In context free languages semantics can be determined from syntax alone

”She was sitting on a fly.” ???

“Fly” has (at least) two different meanings.

Programming languages must be (almost context free)

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(Context Free) Grammars

• Context free grammars are used to describe the syntax of languages:

• Definition:• A set of terminal symbols• A set of non-terminal symbols.• A set of productions (or rules)

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Example 4.1P -> a, P -> bQ -> 0, Q -> 1R -> PQS -> R, S -> Rc

Terminals: a, b, 0, 1, c.Non-terminals (or meta symbols): P, Q, R and S.S is the defining meta symbol of the language:

S = {a0,a1,b0,b1,a0c,a1c,b0c,b1c}

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Syntax tree for “a0c”S 

R   P Q  

a 0 c

Exercise 4.1Draw the syntax tree for b0c using the grammar from example 4.1.

P -> a, P -> bQ -> 0, Q -> 1R -> PQS -> R, S -> Rc

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Backus-Naur-Form (BNF)

C -> 0, C -> 1, ..., C -> 9U -> C, U -> CU

 <digit> ::= 0│1│2│3│4│5│6│7│8│9<unsigned integer> ::= <digit>

│<digit><unsigned integer>

More readable?

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BNF

• <…> - non-terminals in sharp brackets• Terminals are written directly• ::= production• | alternatives

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EBNF (extended BNF)The grammar  

<digit> ::= 0│1│2│3│4│5│6│7│8│9<unsigned integer> ::= <digit>│<digit><unsigned integer>

 may in EBNF be written as 

<unsigned integer> ::= <digit>{<digit>}

Curly brackets means zero or more,square brackets zero or one. 

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Break for exercises:

Exercise 4.2Extend the grammar from example 4.2, so signed integers become valid.

Exercise 4.3Construct a grammar that defines the set of valid identifiers. An identifier is a sequence of characters beginning with aletter followed be zero or more letters or digits, for example exercise11a.

Exercise 4.4Rewrite the grammar from exercise 4.3 (identifiers) to EBNF.

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Syntax Diagrams

digit

910

Non-terminals in rectangles

Terminals in “roundangles”

You always follow the arrows

Exercise 4.5:Draw syntax diagrams for the grammar in exercise 4.2 (identifiers).

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The Syntax definition of Java• http://java.sun.com/docs/books/jls/third_edition/html/j3TOC.html

– Choose:• Java Developer Resources• Documentation • The Java Language Specification

IfThenStatement:if ( Expression ) Statement

Non-terminals in italicTerminals in

fixed width

Productions: new line and indentation

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ExercisesExercise 5.1Write the grammar from exercise 4.2 (signed integers) in the Java notation.

Exersice 5.2Construct a syntax tree for the following Java statement (use the above link to the grammar).

if(x==100)y= 1.1*x;

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Syntax Diagrams for Java

?

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Exercise 5.3In 5.1 the grammar for signed integers was given:

SignedInteger:Signopt Digits

Sign: one of+ -

Digits:DigitDigits Digit

Digit:0NonZeroDigit

NonZeroDigit: one of1 2 3 4 5 6 7 8 9

Draw the corresponding syntax diagrams.

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Exercise 5.4Draw syntax diagrams which describe the Java While-statement.