Part 3 - Syntax, Semantics and Pragmatics

8/18/2019 Part 3 - Syntax, Semantics and Pragmatics

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Three aspects of language

• Syntax is the required grammar and punctuation of the language – Compile-time errors are syntax errors

• Semantics is all about meaning--what thestatements do, what the programs do – Logic errors are semantic errors

• Pragmatics has to do with what’s good! andbad! about a language or program


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Syntax examples

• "#$%$&' statements are one per line(modern languages are free-format

• Pascal uses semicolons between statements(C uses semicolons after statements

• Pascal uses begin…end to group statements(C uses { and }

• Pascal uses the )eyword integer ( C uses int


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Another syntax example

• C* if (x > y) x = x-1; else y--;Pascal* if x > y then x := x-1 else y := y-1;

• +ifferences* parentheses around x > y, theword then, = or :=, semicolon before else, --

• %hese are syntactic differences( the meaningsare identical


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The importance of syntax

• Correct syntax is ob iously important( if youdon’t get it right, your program won’t run

• n a sense, syntax is tri ial( you learn it, youfix it until it’s right, end of story

• .ut the syntax of a language greatly affects* – how easy it is to write programs – how easy it is to read and understand programs – how easy it is to ma)e hard-to-see syntax errors


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Examples of poor syntax

• n "#$%$&', ariables don’t ha e to bedeclared – %herefore, e ery misspelling is a new ariable

• n Pascal, semicolons go between statements – %herefore, adding a statement to a bloc)

in ol es adding a semicolon to the pre ious line


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An example of good syntax

• n &da, control statements ha e the formif…end if, while…end while, case…endcase , etc/ – %his helps a oid the confusion 0in C1 resulting

from large groups of anonymous closing braces

• 2yntax is usually more important forreading and understanding programs thanfor writing them


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Why syntax matters

• %he C ersion has a bug that almost ne er occurs in &da

C* if (x < y) te ! = x; x = y; y = te !;

&da* if x < y then te ! := x; x := y; y := te !

end if;


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Semantics

• 2emantics has to do with the meaning ofconstructs in a language, and the meaningsof programs written in that language

• 2emantics is fundamental to e erything youdo in a language

• 2yntax is 3ust the code! you use todescribe the semantics


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High-level semantics

• 2emantics can affect things at a ery highle el* – C is a procedural language( you describe a set

of procedures to follow – 4a a is an ob3ect-oriented language( you

describe ob3ects and their beha iors – Prolog is a logic language( you describe facts

and the logical relationships among them


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Low level semantics

• 2emantics can affect things at a ery low le el* – C* d" { x = #$x; } while (x < 1%%);

Pascal* re!eat x := #$x &ntil x >= 1%%;

• 'otice that the sense of the test is different* C

exits the loop when the condition becomesfalse, Pascal when it becomes true


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Syntax supports semantics

• & language cannot ha e semantics withoutsyntax to support those semantics

• C couldn’t ha e a f"r loop without syntax• 4a a couldn’t ha e ob3ects without syntax

for creating and using them• This doesn t mean that f"r loops and

ob!ects are syntactic constructs"


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Syntax is typographical

• 2yntax describes the way we write programs asa sequence of characters

• 2yntax can be precisely and formally defined by .'" 0.ac)us-'aur "orm1

• & language in the usual sense is a sequence ofcharacters 0or sounds1 and requires syntax

• .5% you can do many language-li)e thingswith a 65 and no real syntax


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Semantics is fundamental

• 2emantics affects the ery way we thin) about programming

• 2omeone once said, 7ou can write a "#$%$&' program in any language/! – %his is a poor way to program – 7ou can use a language, or you can fight with it – f you are fighting with a language, you are either

• using the wrong language, or • using the language wrong


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Thin#ing in the language

• n C, functional decomposition is the preferred way to write a program

• n 4a a, functional decomposition is one ofthe worst ways to write a program

• n 4a a, you need to* – Choose your set of ob3ects carefully – +ecide the beha iors of each )ind of ob3ect – +ecide how ob3ects communicate and interact


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$oals of this course

• & primary goal of this course is to introduceyou to different ways of thin)ing about

programming• t is essential to understand the genius! of a

language, and to wor) with it, not against it

• 7ou ha e to learn the syntax of a languagewell enough that you can concentrate on thesemantics


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%ragmatics

• Pragmatics has to do with how well thelanguage connects to the real world!

• 2emantics supports pragmatics* some )indsof languages are better for some )inds of

problems• %he choice of a language should depend on

pragmatic considerations


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Examples of pragmatics

• C is fast because it does so little error chec)ing• 4a a programs are less buggy because they

spend so much time on error chec)s• Perl is good for C6 scripts because it has

powerful tools for string processing• 4a a is a better choice for me than C88

because )now 4a a better

Part 3 - Syntax, Semantics and Pragmatics

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