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CPSC 449 Principles of Programm

ing Languages Jörg D

enzinger

1. IntroductionLanguage

Semantics

Parsing

Gramm

ar

Syn

tax

Co

mpiler

Interpreter

?

l-Ca

lcu

lus

SLD-Resolution

abstract data types

loo

ps

Error handling



enzinger

Hum

an LanguagesBabel, m

anyyears ago

Good

Day!G

utenT

ag!

Bon

Jour!



enzinger

Language: definition

Language:1 a: the w

ords, their pronunciation, and the methods

of combining them

used and understood by a largegroup of peopleb (1): audible, articulate, and m

eaningful sound asproduced by the action of the vocal organs(2): a system

atic means of com

municating ideas by

signs or marks w

ith understood meanings (sign

language)



enzinger

Language: definition (cont.)

2 a: form or m

anner of verbal expression; esp. STYLE(forceful language)b: the w

ords and expressions of a particular group orfield (the language of m

edicine)3: the study of language especially as a school subject(O

ld French, from langue "tongue, language:, from

Latin lingua)

Webster's N

ew Encyclopedic D

ictionary, 1996 Edition



enzinger

How

is language supposed to work?

Chair?



enzinger

How

is language supposed to work?

Chair



enzinger

But:



enzinger

So:Chair SyntaxSem

antics



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What do w

e want to com

municate

using a language?

nThingsl

visible/invisiblel

real/imaginary

nConcepts

nA

ctionsn

Relationshipsl

Causall

Temporal

n…



enzinger

Language is more than w

ords!

nLetters are used to form

words

nW

ords are used to form phrases

nPhrases form

sentencesn

Sentences form paragraphs

nParagraphs form

sectionsn

Sections together form chapters

nChapters form

books



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But:

nN

ot every sequence of letters is an "allowed" w

ordn

Not every com

bination of words is a phrase

nN

ot every collection of phrases is a sentence

F Syntax defines w

hat is allowed and w

hat not!But: syntax m

ust be comm

unicated between hum

ans!F

Gram

mars

are used to describe syntax (using rules, laws, etc.)



enzinger

How

do we go from

syntax tosem

antics?

nH

umans learn about the m

eaning of words from

other humans and develop their ow

n relationsbetw

een syntactical constructs and semantical

conceptsn

There are then groups of human beings that have

rather similar sem

antical concepts for the same

syntactical constructF

A language is born



enzinger

Problems w

ith human languages (I)

nSeveral different syntactical constructs can expressthe sam

e semantical concepts:

lChair vs. object you sit on

lThis allow

s poets to exist!n

One syntactical construct can be associated w

ithseveral sem

antical concepts

lA

mbiguity: source for m

any jokes

Chair



enzinger

Problems w

ith human languages (II)

nH

uman languages change

CHA

TILLON

Philip of France, in right and true behalfO

f thy deceased brother Geffrey's son,

Arthur Plantagenet, lays m

ost lawful claim

To this fair island and the territories,To Ireland, Poictiers, A

njou, Touraine, Maine,

Desiring thee to lay aside the sw

ordW

hich sways usurpingly these several titles,

And put these sam

e into young Arthur's hand,

Thy nephew and right royal sovereign.

The Life and D

eath of King John

William

Shakespeare



enzinger

What about com

puter programm

inglanguages?

nFortran

nProlog

nLisp

nC / C++

nJava

nSm

alltalkn

…



enzinger

Why do w

e have the Babel syndrome

here?n

People want to com

bine certain features of otherlanguages into one language

nPeople w

ant to work on a higher abstraction level

and therefore produce a language with such higher

conceptsn

People want to have a language that supports certain

types of tasks:l

Self modifying program

sl

Real-time applications

lVerification of program

sl

etc.n

There is a new idea of com

putability



enzinger

How

is a programm

ing languagesupposed to w

ork?

?



enzinger

How

is a programm


ork?

?Sourcecodefile

Compiler

Inter-preter

mem

ory

prozessor

Compiler

Exe-cutable



enzinger

How

is a programm


ork?

?Sourcecodefile

Compiler

Inter-preter

mem

ory

prozessorInter-preter



enzinger

How

is a programm


ork?

!Sourcecodefile

Compiler

Inter-preter

mem

ory

prozessorInter-preter



enzinger

But still:



enzinger

So, what defines sem

antics?

nLanguage report in natural language

nO

perating system of underlying com

putern

Understanding of designer &

programm

er ofcom

piler/interpreter of language report and machine

specifications



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What do w

e want to com

municate

using a programm

ing language?

nD

ata and the structure of datan

Many different m

anipulations of datan

Sequences and more com

plex combinations of data

manipulations

nA

ccess to external devices, like sensors, files, printers



enzinger

How

do we define a program

ming

language? (I)

Syntax:n

Formal gram

mars like

lContext-free gram

mars

lBackus-N

aur forml

Attribute gram

mars

nSyntax diagram

s



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How

do we define a program

ming

language? (II)

Semantics:

nTheoretically using:l

Operational sem

anticsl

Axiom

atic semantics

lD

enotational (or fixpoint) semantics

nPractically:l

(in theory) by a language reportl

(in practice) by a compiler/interpreter



enzinger

Problems w

ith programm

inglanguages (I)

nSeveral different syntactical constructs can result inthe sam

e computation result:

j:= 0

j:= 0

for i:=1 to 5 do

i:= 1

j := j+

1;w

hile i<>

6 dobeginj:=

j+1;

i:=i+

1end



enzinger

Problems w

ith programm

inglanguages (II)

nProgram

ming languages can change!

Remem

ber the Java 1.1 to Java 1.2 fiasco

But: the same syntactical construct should never have

different meanings

(now, w

e just have to tell the compiler guys this J

)

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