Programming Languages Tucker and Noonan

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Programming LanguagesTucker and Noonan

• Chapter 7 Semantics
• 7.1 Motivation
• 7.2 Expression Semantics
• 7.3 Program State
• 7.4 Assignment Semantics
• 7.5 Control Flow Semantics
• 7.6 Input/Output Semantics
• 7.7 Exception Handling Semantics

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Motivation

• To provide an authoritative definition of the
  meaning of all language constructs for
• Programmers
• Compiler writers
• Standards developers
• A programming language is complete only when its
  syntax, type system, and semantics are
  well-defined.

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Semantics

• Semantics is a precise definition of the meaning
  of a syntactically and type-wise correct program.
• Three approaches
• Operational semantics
• The meaning attached by compiling using compiler
  C and executing using machine M. Ex Fortran on
  IBM 709.
• Direct and straightforward program meaning
• Axiomatic semantics
• Axiomatize the meaning of statements -- Chapter
  12
• Exploration of formal properties of programs
• Denotational semantics
• Statements as state transforming functions
• High-level mathematical precision of program
  meaning
• This chapter uses an informal, operational model.

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Expression Semantics

• Notation Expression tree
• Meaning
• Mathematics (a b) - (c d)
• Polish prefix notation
• - a b c d
• Preorder traversal
• Polish postfix notation
• a b c d -
• Postorder traversal
• Cambridge Polish
• (- ( a b) ( c d))
• Operator precedes operands
• Parentheses

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Infix Notation

• Mathematics meaning
• (a b) - (c d)
• Uses associativity and precedence to disambiguate
• Associativity of Operators
• Language - / Unary -
  ! lt ...
• C-like Left Right Left
• Ada Left non non non
• Fortran Left Right Right Left
• Meaning of a lt x lt b in C-like
• a lt x x lt b ?
• If (a lt x) 1 lt b else 0 lt b

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Precedence of Operators

• Operators C-like Ada Fortran
• Unary - 7 3 3
• 5 5
• / 6 4 4
• - 5 3 3
• ! 4 2 2
• lt lt ... 3 2 2
• Unary not 7 2 2
• Logical and 2 1 1
• Logical or 1 1 1

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Short Circuit Evaluation

• a and b evaluated as
• if a then b else false
• a or b evaluated as
• if a then true else b
• Benefits
• Efficiency
• Shorter code
• Clearer code
• Example in C-like
• x lt Math.pow(y, 3) b bad?
• xlt1 (yy gt 100 ygtx) good?

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More Example

• Node p head
• while (p ! null p.info ! key) p p.next

• while (p ! null ! found)
• if (p.info key) break
• else p p.next
• // using break

• boolean found false
• while (p ! null ! found)
• if (p.info key) found true
• else p p.next
• // avoiding break

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Expression Meaning

• Number precision
• data type size
• (ab)c a(bc) ?
• Side effect
• A change to any non-local variable or I/O.
• What is the value of
• i 2 b 2 c 5
• x b i c i // 19
• y c i b i // 14
• Consider
• y x
• y x1

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Program State

• The state of a program is the collection of all
  active objects and their current values.
• Two maps
• Active objects to specific memory locations
• Active memory locations to specific values.
• State memory ? environment
• The current statement (portion of an abstract
  syntax tree) to be executed in a program is
  interpreted relative to the current state.
• The individual steps that occur during a program
  run can be viewed as a series of state
  transformations.
• For simplicity, ignore the memory location

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Program State Transformation
Program compute the factorial of n
State Transformation

• 1 void main ( )
• 2 int n, i, f
• 3 n 3
• 4 i 1
• 5 f 1
• 6 while (i lt n)
• 7 i i 1
• 8 f f i
• 9
• 10

• n i f
• undef undef undef
• 3 undef undef
• 3 1 undef
• 3, 3, 3 1, 2, 3 1, 2, 6
• 3, 3 1, 2 1, 2
• 3, 3 2, 3 1, 2
• 3, 3 3, 3 2, 6
• 3 3 6

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Assignment Semantics

• Issues
• Multiple assignment
• Example
• a b c 0
• Sets all 3 variables to zero.
• Problems???
• Assignment statement vs. expression
• Copy vs. reference semantics

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Assignment Statement vs. Expression

• In most languages, assignment is a statement
  cannot appear in an expression.
• In C-like languages, assignment is an expression.
• Example if (a 0) ... // an error
• while (p q) // strcpy
• while (ch getc(fp)) ... // ???
• while (p p-gtnext) ... // ???

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Copy vs. Reference Semantics

• Copy a b
• a, b have same value.
• Changes to either have no effect on other.
• Used in imperative languages.
• Reference
• a, b point to the same object.
• A change in object state affects both
• Used by many object-oriented languages.

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Control Flow Semantics

• Turing complete
• a programming language is Turing Complete if its
  program are capable of computing any computable
  function
• To be complete, an imperative language needs
• Statement sequencing
• Conditional statement
• Looping statement

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Sequence

• Format
• s1 s2
• Semantics in the absence of a branch
• First execute s1
• Then execute s2
• Output state of s1 is the input state of s2

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Conditional

• Format
• IfStatement ? if ( Expression ) Statement
  else Statement
• Semantics
• If the value of Expression is true, the meaning
  is the state resulting from evaluating Statement1
  in the current state. Otherwise the meaning is
  the state resulting from evaluating Statement2 in
  the current state
• Example
• if (a gt b)
• z a
• else
• z b

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Various Conditional Statements

• if ltEgt then ltSgt if ltEgt is true then execute ltSgt
• if ltEgt then ltSgt else ltS2gt if ltEgt is true then
  execute ltSgt, otherwise execute ltS2gt
• Dangling-else Ambiguity. Ways to avoid ambiguity
• Syntax the else-clause associated with the
  closest then e.g. C, Java
• if ltE1gt then if ltE2gt then ltS1gt else ltS2gt
• Use compound brackets
• A if ltE1gt then if ltE2gt then ltS1gt else ltS2gt
• B if ltE1gt then if ltE2gt then ltS1gt else ltS2gt
• Avoid use of nesting in then parts
• A if !ltE1gt then ltS2gt else if ltE2gt then ltS1gt
• B if ltE1gt ltE2gt then ltS1gt else if ltE1gt then
  ltS2gt

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Multiple Branches

• Multiple branches case (selection) use the
  value of an expression to select one of several
  substatements for execution
• Pascal, Modula-2 case x of
• ltvalue1gt ltstatements1gt
• ltvalue2gt ltstatements2gt
• else ltstatementsgt
• end
• Ada case x is
• when ltvalue1gt gt ltstatements1gt
• when ltvalue2gt gt ltstatements2gt
• others gt ltstatementsgt
• C/C, Java Notice the use of break
• switch (x)
• case ltvalue1gt ltstatements1gt
• break
• case ltvalue2gt ltstatements2gt
• break

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Multiple Branches

• Points
• Cases can appear in any order, but
  else/others/default must be the last one
• The case value of x is discrete, such as 1, 10,
  6, 9
• The case values must be distinct
• Some languages allow ranges of case values, such
  as in Modula-2, 0..9 represents the value
  either 0, or 1, , or 9 of x
• Case can be replaced with ifthenelse statements
  
• if x ltvalue1gt then ltstatements1gt
• else if x ltvalue2gt then ltstatements2gt
• else ltstatementsgt

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Loops

• Format
• WhileStatement ? while ( Expression ) Statement
• Semantics
• The expression is evaluated.
• If it is true, first the statement is executed,
• and then the loop is executed again.
• Otherwise the loop terminates.

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General Loop Structure

• General structure
• Do S1
• If C exit
• Otherwise Do S2
• Repeat

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Loop Implementation
General loop
While loop
Repeat loop
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For Loop Implementation

• For statement definite iteration for each
  element do
• General form
• for (ltvargt ltinitialgt ltstepgt ltterminationgt)
  do statements
• ltvargt varies from the initial value to the
  termination, each loop varies with a stepwise
• Step can be omitted, default value is 1
• Stepwise can be going either up from small
  initial to large termination, or down from large
  initial to small termination
• Pascal
• for x low to high do S
• for x high downto low do S
• Fortran/Algol
• for x start step increment until end do S
• increment may be negative to implement going
  down
• Ada
• for x in low..high loop S end loop
• for x in reverse low..high loop S end loop
• C/C/Java
• for (e1 e2 e3) S

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For Loop Implementation

• Some languages allow the stepwise to be real or
  other data types
• Fortran/Algol allow real steps such as 0.1, 0.2
• Pascal/Modula-2 allow step type of enumeration
• for x in d do S where d is a type of enumeration
• Perl allows enumeration in a list
• foreach x (_at_list) S
• Java allows enumeration in a collection
• Syntax
• foreach (Type variable collection) statement
• Example
• String names
• foreach (String name names)
• do something with name

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Jump Statements

• Goto jump to a specific statement (labeled by a
  label). Recall jmp, jp in assembly languages
• makes the program difficult to trace, hard to
  prove
• violates the structured programming style
• destroy the system if goto the kernel of the OS
• Break (C/C, Java) limited use of Goto as an
  exit of a loop or selection
• Used inside a loop exit the loop
• Used inside a selection (case) exit the case
• Exit in Ada and Modula-2
• Continue(C/C, Java, Fortran) limited use of
  Goto
• Begin the next loop, ignore the statements after
  it in the body

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7.6 Input/Output Semantics

• Binding open, close
• Access sequential vs. random
• Stream vs. fixed length records
• Character vs. binary
• Format
• I/O Error handling

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Standard Files

• Unix stdin, stdout, stderr
• C stdin, stdout, stderr
• C cin, cout, cerr
• Java System.in, System.out, System.err

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Input/Output Streams

• Fortran
• integer i, a(8)
• write(8,) Enter 8 integers
• read(,) a
• write(,) a
• Java
• file, pipe, memory, url
• filter
• reader, writer

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Formats

• C
• Codes d, e, f, c, s (decimal. float, float,
  char, string)
• Specifier opt-width code
• Ex s 5d 20s 8.2f
• Fortran
• Codes i, f, a (integer, float, string)
• Specifier op-repeat code width
• Ex 8i4, f8.2, a20

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Exception Handling Semantics

• Exception
• an error condition occurring from an operation
  that cannot be resolved by the operation itself
• Purposes
• Simplify programming
• Make applications more robust continue to
  operate under all concevable error situations

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Exception Handling Basic Structure

• Exception events abnormal
• event that the system can not deal with so that
  leave to the programmer, such as
• division by zero,
• get value from a null pointer
• read a file that doesnt exist
• Exception handlers
• Deal with the exception events, such as
• output error
• use default value
• stop the execution

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Exception Handling ProgramStructure

• Ada
• begin
• exception
• when ltexception1gt gt exception1-handler
• when ltexception2gt gt exception2-handler
• .
• end

C/Java try . catch (ltexception1gt)
exception1-handler catch (ltexception2gt)
exception2-handler finally .
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Raise/Throw/Catch Exceptions

• Raise/throw exception
• Implicitly throw built-in method/function throw,
  e.g. division, file access
• Explicitly throw
• throw ltexceptiongt --- C/Java
• raise ltexceptiongt --- Ada
• Catch exception
• If there exists a catch clause to catch a
  specific exception, the exception is handled by
  the corresponding handler. If no handler exists,
  the exception is raised/thrown to the caller. To
  implement this, the throws must be specified in
  the method prototype (signature). E.g. in Java
• public void method_sample() throws ltexceptiongt
  
• throw ltexceptiongt

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Throwing an Exception Figure 7.13
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Exception Post-process

• Termination model
• C/Java, Ada only enter the exception handler
  and execute the finally clause, if any.
• Resumption model
• Eiffel can use the retry statement to re-execute
  the procedure/function in which the exception
  occurred

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Exception Handling Figure 7.9
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Declare Exceptions

• Predefined exceptions (built-in exception)
• User-defined exception e.g. in Java
• Public class MyException extends
  RuntimeException

Creating a New Exception Class
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Java Exception Type Hierarchy Figure 7.10
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Missing Argument Exception
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Invalid Input Exception Figure 7.12
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AssertException Class
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Assert Class Figure 7.15
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Using Asserts