Programming Languages

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ProgrammingLanguages
CSCI-4430 CSCI-6969January 18, 2008

• David Goldschmidt, Ph.D.
• Computer Science
• The College of Saint Rose

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Syntax and Semantics (i)

• Syntax is the form or structure of the
  expressions, statements, and program units of a
  given language
• Syntax of a Java while statement
• while ( ltboolean_exprgt ) ltstatementgt
• Partial syntax of an if statement
• if ( ltboolean_exprgt ) ltstatementgt

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Syntax and Semantics (ii)

• What is semantics?
• The meaning of the expressions, statements, and
  program units of a given language
• Semantics of the given Java while statement?
• while ( ltboolean_exprgt ) ltstatementgt
• Execute ltstatementgt repeatedly (0 or more times)
  as long as ltboolean_exprgt evaluates to true

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Syntax and Semantics (iii)

• Syntax and semantics together define a
  programming language

syntax
programming language
semantics
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Describing Syntax

• Terminology to describe syntax
• A sentence is a string of characters over some
  alphabet
• A language is a set of sentences
• A lexeme is the lowest-level syntactic unit of a
  language (e.g. , sum, while)
• One step above individual characters
• A token is a category of lexemes
• e.g. identifier, equal_sign, multiplication_sign,
  integer_literal, etc.

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Recognizers and Generators (i)

• Languages defined by recognition and generation
• A recognizer reads input strings and determines
  whether the strings belong to the language
• Syntax analysis part of a compiler

language L recognizer
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Recognizers and Generators (ii)

• Languages defined by recognition and generation
• A generator produces syntactically acceptable
  strings of a given language
• Inspect generator rules (i.e. a grammar) to
  determine if a sentence is acceptable for a given
  language

language L generator
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Grammars

• Linguist Noam Chomsky (mid-1950s) developed four
  classes of generative devices or grammars to
  define four classes of languages
• Context-free grammars proved useful for
  describing the syntax of programming languages
• Regular grammars proved useful for describing the
  tokens of a programming language

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Backus-Naur Form (i)

• John Backus and Peter Naur developed a formal
  notation for specifying programming language
  syntax (in 1959/1960)
• Backus-Naur Form (BNF ) nearly identical to
  Chomskys context-free grammars
• Syntax of assignment statement in BNF
• Example sentence whose syntax matches the rule

ltassigngt ? ltvargt ltexpressiongt
total cost quantity
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Backus-Naur Form (ii)

• RHS consists of abstractions (non-terminals), as
  well as lexemes and tokens (terminals)

ltassigngt ? ltvargt ltexpressiongt
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Backus-Naur Form (iii)

• Example language

ltprogramgt ? begin ltstmtsgt end ltstmtsgt ? ltstmtgt
ltstmtgt ltstmtgt ltstmtgt ? ltvargt ltexprgt ltvargt ? a
b c d ltexprgt ? lttermgt lttermgt lttermgt -
lttermgt lttermgt ? ltvargt literal-integer-value
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Derivations (i)

• A derivation is a repeated application of rules
• Starts with the start symbol and ends with a
  sentence
• Many (often infinite!) possible derivations

ltprogramgt gt begin ltstmtsgt end gt begin
ltstmtgt end gt begin ltvargt ltexprgt end
gt begin b ltexprgt end gt
begin b lttermgt lttermgt end gt begin
b ltvargt lttermgt end gt begin b c
lttermgt end gt begin b c 12 end
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Derivations (ii)

• Every string of symbols in the derivationis a
  sentential form
• A sentence is a sentential form that hasonly
  terminal symbols
• Thus terminating the derivation

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Leftmost Derivations (i)

• A leftmost derivation is one in which the
  leftmost non-terminal in each sentential form is
  the one that is expanded next

ltprogramgt gt begin ltstmtsgt end gt begin
ltstmtgt end gt begin ltvargt ltexprgt end
gt begin b ltexprgt end gt
begin b lttermgt lttermgt end gt begin
b ltvargt lttermgt end gt begin b c
lttermgt end gt begin b c 12 end
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Leftmost Derivations (ii)
ltassigngt ? ltvargt ltexprgt ltvargt ? A B C
D ltexprgt ? ltvargt ltexprgt ltvargt
ltexprgt ( ltexprgt ) ltvargt
grammar
what about a rightmost derivation?
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Working with Grammars
ltSgt ? ltAgt ltBgt ltCgt ltAgt ? a ltAgt a ltBgt ? b ltBgt
b ltCgt ? c ltCgt c

• Given this grammar
• Which of the following sentences are generated by
  this grammar?
• baabbcc
• abc
• bbaa
• aabbbbccc

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Reading Assignments

• Read for next week
• Chapters 3