UNIT I Programming Language Syntax and semantics. Kainjan Sanghavi

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1 UNIT I Programming Language Syntax and semantics B y Kainjan Sanghavi

2 Contents Language Definition Syntax Abstract and Concrete Syntax Concept of binding

3 Language Definition Should enable a person or computer program to determine i. If a purported program is valid ii. If it is valid then what its meaning or effect is i.e Syntax and Semantics

4 SYNTAX Set of rules that define the form of a language They define how sentences may be formed as sequences of basic constituents called words. Using these rules we can tell whether a sentence is legal or not. The syntax does not tell us anything about the content (or meaning) of the sentence.

5 SYNTAX As an example, C keywords (such as while, do, if, else,...), identifiers, numbers, operators,... are words of the language. The C syntax tells us how to combine such words to construct wellformed statements and programs. The syntax of a language is defined by two sets of rules: lexical rules and syntactic rules.

6 Lexical Rules Lexical rules specify the set of characters that constitute the alphabet of the language and the way such characters can be combined to form valid words. For example, Pascal considers lowercase and uppercase characters to be identical, but C and Ada consider them to be distinct. Thus, according to The lexical rules, Memory and memory refer to the same variable in Pascal, but to distinct variables In C and Ada.

7 Lexical Rules The lexical rules also tell us that <> (or ) is a valid operator in Pascal but not in C, where the same operator is represented by!=. Ada differs from both, since not equal is represented as /=; delimiter <> (called box ) stands for an undefined range of an array index.

8 Syntactic Rules Syntactic rules specify actual set of rules constituting the set of characters.

9 SYNTAX.. How does one define the syntax of a language? FORTRAN was defined by simply stating some rules in English. ALGOL 60 was defined with a context-free grammar developed by John Backus. This method has become known as BNF or Backus Naur form (Peter Naur was the editor of the ALGOL 60 report.) BNF provides a compact and clear definition for the syntax of programming languages.

10 EBNF definition of a simple programming language syntax rules, <program>::= { <statement>* } <statement>::=<assignment> <conditional> <loop> <assignment>::=<identifier> = <expr> ; <conditional>::= if <expr> { <statement> + } if <expr>{ <statement> +} else { <statement> +} <loop>::= while <expr>{ <statement> +} <expr> ::=<identifier> <number> ( <expr> ) <expr> <operator> <expr>

11 EBNF definition of a simple programming language lexical rules <operator>::= + - * / = < > <identifier>::= <letter> <ld>* <ld>::= <letter> <digit> <number>::= <digit>+ <letter>::= a b c... z

12 SYNTAX TYPES SYNTAX ABSTRACT CONCRETE

13 SYNTAX TYPES Concrete syntax is the surface level of a language (think, strings) Abstract syntax is the deep structure of a language (think, trees/terms) Parsers convert concrete syntax into abstract syntax and have to deal with ambiguity

14 SYNTAX TYPES Abstract syntax: what are the significant parts of the expression? Example: a sum expression has its two operand expressions as its significant parts Concrete syntax: what does the expression look like? Example: the same sum expression can look in different ways: infix (+ 2 3) -- prefix (2 3 +) -- postfix

15 SYNTAX TYPES Concrete syntax is described by contextfree grammars. Abstract syntax can be expressed as algebraic datatypes.

16 SYNTAX TYPES

17 SYNTAX TYPES

18 SYNTAX TYPES

19 SYNTAX TYPES Concrete syntax is described by many ways Abstract syntax can be expressed by only one way algebraic datatypes.

20 Concept of binding A binding in a program is an association of an attribute with a program component such as an identifier or a symbol. For example the data type of the value of a variable is an attribute that is associated with the variable name. The binding time for an attribute is the time at which the binding occurs. For example, in C the binding time for a variable type is when the program is compiled but the value of the variable is not bound until the program executes

21 Binding Times The most common binding times for attributes are (in chronological order): Language definition Language implementation Program translation (compile time) Link edit Load Program execution (run time)

22 Binding Times Language design time: the design of specific program constructs (syntax), primitive types, and meaning (semantics) Language implementation time: fixation of implementation constants such as numeric precision, run-time memory sizes, max identifier name length, number and types of built-in exceptions, etc. Compile time: the time of translation of high-level constructs to machine code and choice of memory layout for objects

23 Link time: the time at which multiple object codes (machine code files) and libraries are combined into one executable Load time: the time at which the operating system loads the executable in memory Run time: the time during which a program executes (runs)

24 Binding Times Attribute Type of a variable identifier Value of a variable identifier General meaning of + Specific meaning of + Meaning of literal (constant) 23 Internal representation of literal 23 Specific computation performed by an external function Value of a "global" data identifier Binding time Translation (Compile) Execution (Run-time) Language definition Translation Language definition Language implementation Link edit Execution

25 Thank You! Angelin

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