Programming Language Concepts CIS 280

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Programming Language Concepts (CIS 280)

• Elsa L Gunter
• NJIT
• Fall 2001

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Bindings of Program Elements

• The binding time is when that selection is made

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Binding Times

• Language definition time language syntax and
  semantics
• Language implementation time interpreter versus
  compiler, aspects left flexible in definition,
  set of available libraries

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Binding Times

• A binding of a program element is the selection
  of a particular value for it from a set of
  possible values
• Compile time data layout, internal data
  structures
• Link time (load time) binding of values to
  identifiers across program modules
• Run time actual values assigned to non-constant
  identifiers

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To class

• In expression x y 5 what bindings can you
  identify, and when are they most likely made?

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Program Aspects

• Syntax what valid programs look like
• Semantics what valid programs mean what they
  should compute
• Compiler must contain both information

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Major Phases of a Compiler

• Lex
• Break the source into separate tokens
• Parse
• Analyze phrase structure and apply semantic
  actions, usually to build an abstract syntax tree
• Semantic analysis
• Determine what each phrase means, connect
  variable name to definition (typically with
  symbol tables), check types

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Major Phases of a Compiler

• Translate to intermediate representation
• Instruction selection
• Optimize
• Emit final machine code

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Major Phases of a Compiler
Source Program
Lex
Relocatable Object Code
Instruction Selection
Tokens
Linker
Parse
Unoptimized Machine-Specific Assembly Language
Abstract Syntax
Machine Code
Semantic Analysis
Optimize
Optimized Machine-Specific Assembly Language
Symbol Table
Translate
Emit code
Intermediate Representation
Assembly Langague
Assembler
Modified from Modern Compiler Implementation in
ML, by Andrew Appel
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Example of Optimization

• Program code X Y Z W
• Load reg1 with Y
• Load reg2 with Z
• Add reg1 and reg2, saving to reg1
• Store reg1 to tmp
• Load reg1 with tmp
• Load reg2 with W
• Add reg1 and reg2, saving to reg1
• Store reg1 to X
• Eliminate two steps marked

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Language Syntax

• Syntax is the description of which strings of
  symbols are meaningful expressions in a language
• It takes more than syntax to understand a
  language need meaning (semantics) too
• Syntax is the entry point

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Features of a Good Syntax

• Readable
• Writeable
• Lack of ambiguity
• Suggestive of correct meaning
• Ease of translation

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Elements of Syntax

• Character set previously typically ASCII, now
  often 64 character sets (UNICODE)
• Keywords usually reserved
• Special constants cannot be assigned to
• Identifiers can be assigned to
• Operator symbols
• Delimiters (parenthesis, braces,brackets,)
• Blanks (aka white space)

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Elements of Syntax

• Expressions
• Type expressions
• Declarations
• Statements (in imperative languages)
• Subprograms (subroutines)

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Elements of Syntax

• Modules
• Interfaces
• Classes (for object-oriented languages)
• Libraries

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Formal Language Descriptions

• Regular expressions, regular grammars
• Context-free grammars, BNF grammars, syntax
  diagrams
• Finite state automata
• Whole family more of grammars and automata
  covered in automata theory

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Grammars

• Grammars are formal descriptions of which strings
  over a given character set are in a particular
  language
• Language designers write grammar
• Language implementers use grammar to know what
  programs to accept
• Language users use grammar to know how to write
  legitimate programs

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Regular Expressions

• Start with a given character set a,
  b, c
• Build bigger regular expressions from smaller
  ones
• Each character is a regular expression
• It represents the set of one string containing
  just that character

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Regular Expressions

• If x and y are regular expressions, then xy is a
  regular expression
• It represents the set of all strings made from
  first a string in x then a string in y
• If x and y are regular expressions, then x?y is a
  regular expression
• It represents the set of strings in either x or y

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Regular Expressions

• If x is a regular expression, then so is (x)
• It represents the same thing as x
• If x is a regular expression, then so is x
• It represents strings made from concatenating
  zero or more strings from x
• ?
• It represents the empty set

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Example Regular Expressions

• (0?1)1
• The set of all strings of 0s and 1s ending in
  1, 1, 01, 11,
• ab(a)
• The set of all strings of as and bs with
  exactly one b
• ((01) ?(10))
• You tell me
• Regular expressions (equivalently, regular
  grammars) important for lexing, breaking strings
  into recognized words

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Example Lexing

• Regular expressions good for describing lexemes
  (words) in a programming language
• Identifier (a ? b ? ? z ? A ? B ? ? Z) (a ?
  b ? ? z ? A ? B ? ? Z ? 0 ? 1 ? ? 9 ? _ ?
  )
• Digit (0 ? 1 ? ? 9)
• Number (1 ? ? 9)(0 ? ? 9) ? (1 ? ? 9)(0
  ? ? 9)
• Keywords if if, while while,

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Implementing Regular Expressions

• Regular expressions, regular grammars reasonable
  way to generates strings in language
• Not so good for recognizing when a string is in
  language
• Regular expressions which option to choose, how
  many repetitions to make
• Answer finite state automata