Download Chapter 7 - CSUDH Computer Science

Programming Languages
Tucker and Noonan
Chapter 7: Semantics
7.1
7.2
7.3
7.4
7.5
7.6
7.7
Motivation
Expression Semantics
Program State
Assignment Semantics
Control Flow Semantics
Input/Output Semantics
Exception Handling Semantics
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7-1
Motivation
•
To provide an authoritative definition of
the meaning of all language constructs
for:
1. Programmers
2. Compiler writers
3. 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:
• -+ab*cd
• Preorder traversal
– Polish postfix notation
• ab+cd*• 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
C-like
Ada
Fortran
+-*/
Left
Left
Left
Unary Right
non
Right
**
non
Right
== != < ...
Left
non
Left
– Meaning of a < x < b in C-like
• a < x && x < b ?
• If (a < x) 1 < b else 0 < b
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Precedence of Operators
Operators
Unary **
*/
+== !=
< <= ...
Unary not
Logical and
Logical or
C-like
7
6
5
4
3
7
2
1
Ada
3
5
4
3
2
2
2
1
1
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Fortran
3
5
4
3
2
2
2
1
1
7-6
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 < Math.pow(y, 3) || b – bad?
– x<1 && (y*y > 100 || y>x) – good?
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More Example
1.
Node p = head;
while (p != null && p.info != key) p = p.next;
2.
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
3.
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Expression Meaning
• Number precision
– data type size
– (a+b)+c == a+(b+c) ?
• 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 = x+1;
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Program State
• The state of a program is the collection of all active
objects and their current values.
• Two maps:
1. Active objects to specific memory locations
2. 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
1 void main ( ) {
2 int n, i, f;
3 n = 3;
4 i = 1;
5 f = 1;
6 while (i < n) {
7
i = i + 1;
8
f = f * i;
9 }
10 }
State Transformation
n
undef
3
3
3, 3, 3
3, 3
3, 3
3, 3
3
i
undef
undef
1
1, 2, 3
1, 2
2, 3
3, 3
3
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f
undef
undef
undef
1, 2, 6
1, 2
1, 2
2, 6
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->next) ... // ???
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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 > b)
z = a;
else
z = b;
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Various Conditional Statements
• if <E> then <S> : if <E> is true then execute <S>
• if <E> then <S> else <S2>: if <E> is true then execute
<S>, otherwise execute <S2>
• Dangling-else: Ambiguity. Ways to avoid ambiguity
– Syntax: the else-clause associated with the closest then – e.g.
C, Java
if <E1> then if <E2> then <S1> else <S2>
– Use compound brackets
• A:
• B:
if <E1> then { if <E2> then <S1> } else <S2>
if <E1> then { if <E2> then <S1> else <S2> }
– Avoid use of nesting in then parts
• A:
• B:
if !<E1> then <S2> else if <E2> then <S1>
if <E1> && <E2> then <S1> else if <E1> then <S2>
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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
<value1>: <statements1>
<value2>: <statements2>
…
[else: <statements>]
end
Ada:
case x is
when <value1> => <statements1>;
when <value2> => <statements2>;
…
[others => <statements>];
C/C++, Java : Notice: the use of break
switch (x) {
case <value1>: <statements1>;
[break;]
case <value2>: <statements2>;
[break;]
…
[default: <statements>];
}
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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 if—then—else statements
if x == <value1> then <statements1>;
else if x == <value2> then <statements2>;
…
else: <statements>;
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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
S1
C
S2
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Loop Implementation
General
loop
While
loop
Ada:
loop
S1
exit when C
S2
end loop
Ada:
S1
while (!C) loop
S1
end loop
Repeat Ada:
while (!C) loop
loop
S2
end loop
C/C++, Java:
while (t) {
S1
if (C) break;
S2
}
C/C++, Java:
do
S1
while (C)
C/C++, Java:
while (!C) {
S2
}
Pascal:
S1;
while (C) do
begin
S2
S1
end
Pascal:
Perl:
repeat
S1
S1
while (!C) {
until C
S1
}
Pascal:
Perl:
while (!C) do unless (C) {
begin
S2
S2
}
end
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For Loop Implementation
• For statement: definite iteration – for each element do
• General form:
– for (<var> = <initial>; [<step>]; <termination>) do statements
– <var> 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 (@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 doesn’t exist
exception event>
<
Automatically
jump
• Exception handlers
– Deal with the exception events,
such as
<exception handler>
• output error
• use default value
• stop the execution
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Exception Handling ProgramStructure
Ada:
C++/Java:
begin
…
exception
when <exception1> =>
exception1-handler;
when <exception2> =>
exception2-handler;
….
end
try {
….
} catch
(<exception1>) {
exception1handler;
} catch
(<exception2>) {
exception2handler;
}
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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 <exception> --- C++/Java
• raise <exception> --- 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 <exception> {
…
throw <exception>;
…
}
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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 reexecute 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
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