Syntax

1
Syntax Semantic

• Introduction
• Organization of Language Description
• Abstract Syntax
• Formal Syntax
• The Way of Writing Grammars
• Formal Semantic

2
Introduction

• Language Description syntax and semantic
• Syntax how program in language are built up
• Semantic what program mean
• Dates represented by D (digit) and Symbol (/)
• DD/DD/DDDD --gtsyntax01/02/2001 --gtUS
  Jan 2, 2001 Others Feb 1, 2001
• Same syntax, different semantic

3
Organization of Language Description

• Tutorials
• Reference Manuals
• Formal Definition

4
Tutorials
Organization of Language Description

• What the main constructs of the language are
• How they are meant to be used
• Useful examples for imitating and adapting
• Introduce syntax and semantics gradually

5
Reference Manuals
Organization of Language Description

• Describing the syntax and semantics
• Organized around the syntax
• Formal syntax and informal semantic
• Informal semantic English explanations and
  examples to the syntactic rules
• Began with the Algol60 free of ambiguities

6
Formal Definition
Organization of Language Description

• Precise description of the syntax and semantics
• Aimed at specialists
• Attaches semantic rules to the syntax
• Conflicting interpretations from English
  explanation
• Precise formal notation for clarifying subtle
  point

7
Abstract Syntax

• Capture intent, independent of notation
• Intent apply operator to operands a and b
• a b
• ( a b) a b
• ADD a TO b
• The written representation is different, but the
  abstract syntax is the same
• identifies the meaningful components of each
  construct

8
Formal Syntax

• Consists of two layers
• a lexical layer
• a grammar layer
• Lexical syntax the spelling of words
• Grammars the key formalism for describing
  syntax universal programming language
• Example
• if (ltexprgt) ltstatementgt
• if ltexprgt then ltstatementgt

9
Lexical syntax
Formal Syntax

• Tokens/Terminals units in programming language
• Lexical syntax correspondence between the
  written representation (spelling) and the tokens
  or terminals in a grammar
• Keywords alphabetic character sequences unit
  in a language - if , while
• Reserved words keyword that can not be used as
  names

10
Context-Free Grammars
Formal Syntax

• Concrete syntax describes its written
  representation, including lexical details such
  as the placement of keywords and punctuation
  marks
• Context-free grammar or grammars notation for
  specifying concrete syntax

11
Context-Free Grammars (cont.)

• Grammar has four parts
• A set of tokens or terminals atomic symbols
• A set of nonterminals variable representing
  construct
• A set of rules (called productions) identify
  the component of construct
• ltnonterminal gt terminal ltnonterminal gt
• A starting nonterminal represents a main
  construct

12
The Way of Writing Grammars

• The productions are rules for building string
• Parse Trees show how a string can be built
• Notation to write grammar
• Backus-Naur Form (BNF)
• Extended BNF (EBNF)
• Syntax charts graphical notation

13
BNF arithmetic expressions
The Way of Writing Grammars

• ltexpression gt ltexpression gtltterm gt
• ltexpression gt- ltterm gt
• ltterm gt
• ltterm gt ltterm gt ltfactor gt
• ltterm gt/ ltfactor gt
• ltfactor gt
• ltfactor gt number
• name
• ( ltexpression gt )

14
EBNF arithmetic expressions
The Way of Writing Grammars

• ltexpression gt ltterm gt ( - ) ltterm gt
• ltterm gt ltfactor gt ( / ) ltfactor gt
• ltfactor gt ( ltexpression gt )
• name
• number

15
Syntax Chart arithmetic expressions
The Way of Writing Grammars
expression
term

-
term
factor

/
factor
expression
)
(
name
number
16
Parse Tree respect to arithmetic expressions
The Way of Writing Grammars

• number7 number7 - number4 number2 number3

17
Assignment

• Draw a parse tree with respect to the BNF grammar
  for arithmetic expression
• 2 3
• ( 2 3 )
• 2 3 5
• ( 2 3 ) 5
• 2 ( 3 5 )

18
Assignment (cont.)

• Draw parse trees using the following grammar
• S id expr if expr then S
  if expr then S else S
• while expr do s
• begin SL end
• SL S
• S SL
• a. while expr do id expr
• b. begin id expr end
• c. if expr then if expr then S else S

19
Assignment (cont.)

• Write the following grammar in Pascal or C or
  Java by using BNF or EBNF or syntax chart
• program (heading)
• statement
• identifier
• variable

20
Formal Semantic

• Static semantic compile-time properties -
  type correctness, translation- determined from
  the static text of a program, without running
  the program on actual data.
• Dynamic semantic run-time properties
• - value of expression
• - the effect of statement
• - determined by actually doing a computation

21
Semantics or meaning

• Semantic any property of a construct
• The semantic of an expression 23
• Point of view Semantic
• An expression evaluator its value 5
• A type checker type integer
• An infix-to-postfix translator string 2 3

22
Semantic Methods

• Several method for defining semantics
• The approaches are used for different purposes by
  different communities.
• The values of variables a and b in ab depend on
  the environment.
• An environment consists of bindings from variable
  to values.

23
Semantic Methods (cont.)
Formal semantics

• Informal semantics
• Synthesized attributes
• Attribute grammars
• Natural semantics
• Operational semantics
• Denotational semantics
• Axiomatic semantics (or proof rules)

Static semantic Dynamic semantic
24
Synthesized Attributes
Semantic Methods

• Synthesize the meaning of a construct from that
  of its components
• The semantic of a construct any quantities
  associated with the construct
• attribute a quantity associated with a
  construct
• X.a attribute a of X (nonterminal/terminal)
• E.val attribute val of Expression E

25
Synthesized Attributes (cont.)

• To define the meaning of a construct
• Attribute attach to grammar symbols
• Semantic rules attach to production
• Synthesized attribute N.a semantic rules attach
  to productions with N on the left side
• Evaluation order
• information flows bottom-up in a parse tree
• attribute value at a node attributes at the
  children of the node (its subtree)

26
Attribute Grammars
Semantic Methods

• A generalization of synthesized attributes
• The meaning of a construct depend on the
  surrounding context.
• Attribute values can flow up and down a parse
  tree.
• The meaning at a node depend on information from
  the rest of the tree

27
Attribute Grammars (cont.)

• To define the meaning of a construct
• Attribute attach to grammar symbols, each
  attribute designate synthesized or inherited
• Semantic rules attach to productions. If
  nonterminal N appears on the left side of
  production, attach semantic rules defining the
  synthesized attribute of N.If nonterminal A
  appears on the right side of production, attach
  semantic rules defining the inherited attribute
  of A.

28
Natural Semantics
Semantic Methods

• Associated logical rules with the syntax.
• The logical rules use to deduce the meaning of a
  construct.
• Abstract syntax num(val) for numbers
• num is a token and val is its associated value
• axioms rules without conditions
• E v refers to expression E has value v

29
Natural Semantics (cont.)

• Each rule has a name.
• Conditions appear above a line.
• The conclusion appears below the line.
• E num(val) plus E1 E2 times
  E1 E2
• E1 v1 E2 v2 (sum)plus E1 E2 v1 v2

30
Natural Semantics (cont.)

• The value of an expression depend on the values
  of the variable.
• Environments handle variable by binding names
  to values
• Environment object with two operations
• bind(x,v,env) a new envi. that binds variable x
  to value v the binding of all other variables
  are as in the envi. env
• lookup(x,env) the value bound to x in envi env

31
Natural Semantics (cont.)

• The empty envi. nil bind no variables.
• env E vIn envi. env, expression E has
  value v.
• env E1 v1 env E2 v2 (sum)
  env plus E1 E2 v1 v2
• env x lookup(x,env)In envi. env, variable
  x has value lookup(x,env)