CH 2.pptx

Programming Languages
( ECEg4182)
1
UNIT –2
Language Design Issue

Outlines
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 Description of a Language
 Formal Methods of Describing Syntax
 Context-Free Grammars
 Backus-Naur Form

Description of a Language
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 Syntax: the form or structure of the expressions, statements, and program units.
 Syntax is defined using some kind of rules
• Specifying how statements, declarations, and other language constructsare
written.
 Semantics: the meaning of the expressions, statements, and program units.
• What programs do, their behavior and meaning
 Semantics is more complex and involved. It is harder to define, e.g., natural
language .
 Example: if statement
• Syntax: if (<expr>) <statement>
• Semantics: if <expr> is true, execute <statement>

Definitions
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one possible lexeme
 Sentence is a string of characters over some alphabets.
 Language is a set of sentences
 Lexeme is the lowest level syntactic unit of the language (i.e.++, int, total)
 The lexemes of a PL include its numeric literals, operators, and special words…
 Lexemes are partitioned into groups -for example, the names of variables,
methods, classes, and so forth in a PL form a group called identifiers.
 Token is a category of lexemes (e.g. identifier, Keyword, Whitespace…)
 E.g., an identifier is a token that can have lexemes, or instances. In some cases, a token has
only a single possible lexeme. E.g., the token for the arithmetic operator symbol + has just

Definitions…
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 Consider the following Java statement:
index = 2 * count + 17;
• The lexemes and tokens of this statement are
Lexemes Tokens
index
=
2
identifier
equal_sign
int_literal
mult_op
identifier
plus_op
int_literal
semicolon
*
count
+
17
;

Definitions…
6
• Recognizers
– A recognition device reads input strings over the alphabet of the language and decides
whether the input strings belong to the language.
– Example: syntax analysis part of a compiler
– Compilers and Interpreters recognize syntax and convert it into machine understandable form.
• Generators
– A device that generates sentences of a language.
– One can determine if the syntax of a particular sentence is syntactically correct by
comparing it to the structure of the generator.

Formal Description of Syntax
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 Formal language-generation mechanisms, usually called grammars, are
commonly used to describe the syntax of programming languages.
 Most widely known methods for describing syntax:
 Context-Free Grammars ( CFG’s)
 Backus-Naur Form ( BNF) (1959)

BNF and Context-Free Grammars
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•Context-Free Grammars(CFG)
– Developed by Noam Chomsky in the mid-1950s
– Language generators, meant to describe the syntax of natural languages
– Define a class of languages called context-free languages
•Backus-Naur Form (BNF)
– Invented by John Backus to describe the syntax of Algol 58
– Is a formal mathematical way to describe the syntax of the programming languages.
– BNF is equivalent to context-free grammars.

BNF Terminologies
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 BNF is a way of defining syntax. It consists of
 A set of terminal symbols
• Terminals are lexemes or token
A set of non-terminal symbols
 An abstractions that represent classes of syntactic structures
 Syntactic variables that can be expanded into strings of tokens orlexemes
 A set of production rules
 A rule has a left-hand side (LHS), which is a nonterminal, and a right-hand side (RHS),
which is a string of terminals and/or non-terminals
<Left-Hand-Side> => <Right-Hand-Side>

BNF Terminologies…
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 The start symbol is the particular non-terminal that forms the starting point of
generating a sentence of the language.
 A start symbol is a special element of the non-terminals of agrammar.
 Grammar is a finite non-empty set of rules for putting strings together and so
corresponds to a language
 BNF notations
 Non-terminals are denoted by surrounding symbol with <>
 Alternation is denoted by |
Replacement is denoted by =>. These are the productions

BNF Terminologies…
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 Consider the sentence “The dog bites the man”
<sentence> => <subject> <predicate>
<subject> => <article> <noun>
<predicate> => <verb> <direct-object>
<direct-object> => <article> <noun>
<article> =>The | A
<noun> => man| dog
<verb> ::= bits | pets

BNF Rules
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 A rule has a left-hand side (LHS) and a right-hand side (RHS)
 LHS is a single non-terminal.
 RHS contains one or more terminals or non-terminals
 A rule tells how LHS can be replaced by RHS, or how RHS is grouped
together to form a larger syntactic unit (LHS)  traversing the parse tree up
and down
 A non-terminal can have more than one RHS
 A syntactic list can be described using recursion
<ident_list> ident|
ident, <ident_list>

Derivation
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 A derivation is a repeated application of rules, starting with the start symbol and
ending with a sentence (all terminal symbols).
 An Example Grammar
<program> → <stmts>
<stmts> → <stmts> | <stmt>
<stmt> → <var> = <expr>
<var> → a | b | c | d
<expr> → <term> + <term> | <term> - <term>
<term> → <var> | const

Derivation
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<program> => <stmts>
=> <stmt>
=> <var> = <expr>
=> a = <expr>
=> a = <term> + <term>
=> a = <var> + <term>
=> a = b + <term>
=> a = b + const
 This derivation begins with the start symbol, <program>. The symbol => is read “derives.”
Each successive string in the sequence is derived from the previous string by replacing one of
the nonterminals with one of that nonterminal’s definitions.

Derivation
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<assign> => <id> = <expr>
=>
=>
=>
A
A
A
=
=
=
<expr>
<id> * <expr>
B * <expr>
=> A = B * ( <expr> )
=> A = B * ( <id> + <expr> )
=> A = B * ( A + <expr> )
=> A = B * ( A + <id> )
=> A = B * ( A + C )

Derivation
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 Every string of symbols in a derivation is a sentential form
 A sentence is a sentential form that has only terminal symbols
 A leftmost derivation is one in which the leftmost nonterminal in each sentential
form is the one that is expanded
 A derivation may be neither leftmost nor rightmost

Parse Tree Generation
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 These hierarchical structures are called parse trees. For example, the parse tree
 A parse tree gives the structure of the program so semantics of the program is
related to this structure. E.g. local scopes, evaluation order of expressionsetc.
 During compilation, parse trees might be required for code generation, semantic
analysis and optimization phases.
 After a parse tree generated, it can be traversed to do various tasks of
compilation.
 One of the most attractive features of grammars is that they naturally describe the
hierarchical syntactic structure of the sentences of the languages they define.

Parse Trees
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A parse tree for the simple statement
A = B * (A + C)
 Every internal node of a parse tree is labeled
with a nonterminal symbol.
 Every leaf is labeled with a terminal symbol.
 Every sub tree of a parse tree describes one
instance of an abstraction in the sentence.

Ambiguous Grammars
 A grammar that generates a sentential form for which there are two or more distinctparse
trees is said to be ambiguous
A = B + C * A ?
20

Precedence and Grammar…
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nonterminals to represent operands.
 When an expression includes two different operators, for example, x + y * z, one
obvious semantic issue is the order of evaluation of the two.
 This semantic issue can be solved by assigning different precedence levels to
operators.
 The correct ordering is specified by using separate nonterminal symbols to
represent the operands of the operators that have different precedence. This
requires additional nonterminals and some new rules.
 Instead of using <expr> for both operands of both + and *, we could use three

Precedence and Grammar…
 If <expr> is the root symbol for expressions, + can be forced to the top of the
parse tree by having <expr> directly generate only + operators, using the new
nonterminal, <term>, as the right operand of +.
 Next, we can define <term> to generate * operators, using <term> as the left
operand and a new nonterminal, <factor>, as its right operand. Now, * will
always be lower in the parse tree, simply because it is farther from the start
symbol than + in every derivation.
 <term> and <expr> has deferent precedence
 Once in side of <term> there is no way to drive + (only one parse is possib
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le).

Precedence and Grammar
Example

Precedence and Grammar
<assign> => <id> = <expr>
=> A = <expr>
=> A = <expr> + <term>
=> A = <term> + <term>
=> A = <factor> + <term>
=> A = <id> + <term>
=> A = B + <term>
=> A = B + <term> * <factor>
=> A = B + <factor> * <factor>
=> A = B + <id> * <factor>
=> A = B + C * <factor>
=> A = B + C * <id>
=> A = B + C * A
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