syntax: 10/18/2015it 3271 semantics: describe the structures of programs describe the meaning of...

Syntax:

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Semantics:

Describe the structures of programs

Describe the meaning of programs

Programming Languages (formal languages)

-- How to describe them?-- How to use them? (machine and human)

Grammars -- Ambiguous (sometimes)

Textbook, manuals -- Confusing (always) solution: denotation semantics

(for nuts only)

solution: using unambiguous only

English Grammar The man hit the ball. subject verb object

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The man saw the girl with a telescope. subject verb object

The purpose of grammar:

To have a device to generate all valid sentences in the target language (from a root).

To tell whether a sentence is valid.

Chomsky:

(old fashion)

Noam Chomsky1928 -

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http://www.canada.com/nationalpost/news/issuesideas/story.html?id=1385b76d-6c34-4c22-942a-18b71f2c4a44

Syntactic Structures (1957)

Generative Grammar

A valid sentence is generated from a root according to some fixed rules (grammar).

Syntactic Structures

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S

NP VP

T N Verb

the man

the ball

hit

NP

T N

the man hit the ball

Deviation: the sequence of processes that generate a sentence

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<S> <NP> <VP> <T> <N> <VP>the <N> <VP>the man <VP>the man <V> <NP>the man hit <NP>the man hit <T> <N> the man hit the <N> the man hit the ball

<S> ::= <NP> <VP> <NP> ::= <T> <N><VP> ::= <V> <NP><T> ::= the<N> ::= man | ball<V> := hit | took

Grammar 1

the man hit the ball

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Parse: v. To break (a sentence) down into its component parts of speech with an explanation of the form, function, and syntactical relationship of each part.

(American Heritage Dict.)

the dog loves the cat

the loves dog the cat

loves the dog the cat

×

×

A grammar for Arithmetic Expression

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<exp> ::= <exp> + <exp> |

<exp> * <exp> |

( <exp> ) |

a | b | c

Example: ((a+b)*c)

Is this expression valid?

<exp>( <exp> )( <exp> * <exp> )(( <exp> ) * <exp> )((<exp> + <exp> ) * <exp> )((a + <exp> ) * <exp> )((a +b) * <exp> )((a+b)*c)

Yes

A Parse Tree for ((a+b)*c)

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<exp>

<exp> + <exp>

( <exp> )

<exp> * <exp>

( <exp> )

a b

c

Parse Trees for a+b*c

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<exp>

<exp> + <exp>

<exp> * <exp>

a b

c

<exp>

<exp> * <exp>

<exp> + <exp>

b c

a

?

What is the meaning of a+b*c

Restrictions on Grammars

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Unrestricted Grammars(type-0)

Why context sensitive grammars have less restrictions than context free grammars?

Right/Left Linear Grammars(type-3)

Context Sensitive(type-1)

Context Free(type-2)

Diagram in terms of the sizes of the set of restrictions

Chomsky Hierarchy

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Regular Expressions(type-3)

Computable (formal) languages(type-0)

Context-free languages(type-2)

Context-sensitive languages(type-1)

Diagram in terms of the sizes of the language families

• A BNF grammar consists of four parts:

– The finite set of tokens (terminal symbols)

– The finite set of non-terminal symbols

– The start symbol

– The finite set of production rules

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<S> ::= <NP> <VP> <NP> ::= <T> <N><VP> ::= <V> <NP><T> ::= the<N> ::= man | ball<V> ::= hit | took

Grammars in BNF (Backus-Naur Form)

Constructing Grammars

• Using divide and conquer to simplify the job. • Data types, variable names (identifiers)• One variable, one type (this is not grammar’s job to make sure)

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float a;

boolean a, b, c;

int a, b;

<var-dec>

Tokens:

• How is such a program file (a sequence of characters) divided into a sequence of tokens?

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e.g. • identifiers (const, x, fact)

• keywords (if, const)

• operators (==)

• constants (123.4), etc.

• Programs stored in files are just sequences of characters, but we want to prepare them into tokens before further analysis.

Reserved words

Tokens are atoms of the program

Lexical Structure And Phrase Structure

• Grammars so far have defined phrase structure: how a program is built from a sequence of tokens

• We also need to define lexical structure: how a text file is divided into tokens

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Separate Grammars

• Usually there are two separate grammars– to construct a sequence of tokens from a file of

characters (Lexical Structure)

– to construct a parse tree from a sequence of tokens (Phrase Structure)

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<program-file> ::= <end-of-file> | <element> <program-file>

<element> ::= <token> | <one-white-space> | <comment><one-white-space> ::= <space> | <tab> | <end-of-line><token> ::= <identifier> | <operator> | <constant> | …

Separate Compiler Passes

• Scanner tokens string• parser parse tree

• (more to do afterwards)

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Historical Note #1

• Early languages sometimes did not separate lexical structure from phrase structure

– Early Fortran and Algol dialects allowed spaces anywhere, even in the middle of a keyword

– Other languages like PL/I or Early Fortran allow keywords to be used as identifiers

This makes them difficult to scan and parse

It also reduces readability

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Historical Note #2

• Some languages have a fixed-format lexical structure -- column positions are significant

– One statement per line (i.e. per card)

– First few columns for statement label

– Etc.

• Early dialects of Fortran, Cobol, and Basic

• Almost all modern languages are free-format: column positions are ignored

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Other Grammar Forms

• BNF variations

• EBNF variations

• Syntax diagrams

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BNF Variations

• Some use or = instead of ::=

• Some leave out the angle brackets and use a distinct typeface for tokens

• Some allow single quotes around tokens, for example to distinguish ‘|’ as a token from | as a meta-symbol

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Sir, please Step away from the ASR-33

Interesting operator!!Or not!

http://queue.acm.org/detail.cfm?id=1871406

EBNF Variations

• Additional syntax to simplify some grammar chores:

– {x} to mean zero or more repetitions of x

– [x] to mean x is optional (i.e. x | <empty>)

– () for grouping

– | anywhere to mean a choice among alternatives

– Quotes around tokens, if necessary, to distinguish from meta-symbols

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EBNF Examples

• Anything that extends BNF this way is called an Extended BNF: EBNF

• There are many variations

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<stmt-list> ::= {<stmt> ;}

<if-stmt> ::= if <expr> then <stmt> [else <stmt>]

<thing-list> ::= { (<stmt> | <declaration>) ;}

Syntax Diagrams

• Syntax diagrams (“railroad diagrams”)

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if then elseexpr stmt stmtif-stmt

<if-stmt> ::= if <expr> then <stmt> else <stmt>

Bypasses

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if then elseexpr stmt stmtif-stmt

<if-stmt> ::= if <expr> then <stmt> [else <stmt>]

Branching

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exp

exp + exp

exp * exp

( exp )

a

b

c

<exp> ::= <exp> + <exp> | <exp> * <exp> | ( <exp> )| a | b | c

Loops

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<exp> ::= <addend> {+ <addend>}

exp addend

+

Syntax Diagrams, Pro and Con

• Easier for human to read (follow)

• Difficult to perceive the phrase structures (syntax tree)?

• Harder for machine to read (for automatic parser-generators)

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Conclusion• We use grammars to define programming

language syntax, both lexical structure and phrase structure

• Connection between theory and practice– Two grammars, two compiler passes– Parser-generators can produce code for those

two passes automatically from grammars (compiler tools)

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syntax: 10/18/2015it 3271 semantics: describe the structures of programs describe the meaning of...

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