Theory of programming language chapter 5

ISBN 0-321-49362-1
Chapter 5
Names, Bindings,
and Scopes

Copyright © 2018 Pearson. All rights reserved. 1-2
Chapter 5 Topics
• Introduction
• Names
• Variables
• The Concept of Binding
• Scope
• Scope and Lifetime
• Referencing Environments
• Named Constants

Introduction
• Imperative languages are abstractions of von
Neumann architecture
– Memory
– Processor
• Variables are characterized by attributes
– To design a type, must consider scope, lifetime,
type checking, initialization, and type compatibility

Names
• Design issues for names:
– Are names case sensitive?
– Are special words reserved words or keywords?

Names (continued)
• Length
– If too short, they cannot be connotative
– Language examples:
• C99: no limit but only the first 63 are significant; also,
external names are limited to a maximum of 31
• C# and Java: no limit, and all are significant
• C++: no limit, but implementers often impose one

Names (continued)
• Special characters
– PHP: all variable names must begin with dollar
signs
– Perl: all variable names begin with special
characters, which specify the variable’s type
– Ruby: variable names that begin with @ are
instance variables; those that begin with @@ are
class variables

Names (continued)
• Case sensitivity
– Disadvantage: readability (names that look alike
are different)
• Names in the C-based languages are case sensitive
• Names in others are not
• Worse in C++, Java, and C# because predefined
names are mixed case (e.g.
IndexOutOfBoundsException)

Names (continued)
• Special words
– An aid to readability; used to delimit or separate
statement clauses
- A keyword is a word that is special only in
certain contexts
– A reserved word is a special word that cannot be
used as a user-defined name
– Potential problem with reserved words: If there
are too many, many collisions occur (e.g., COBOL
has 300 reserved words!)

Variables
• A variable is an abstraction of a memory cell
• Variables can be characterized as a six tuples
of attributes:
– Name
– Address
– Value
– Type
– Lifetime
– Scope

Variables Attributes
• Name - not all variables have them
• Address - the memory address with which it is
associated
– A variable may have different addresses at different times
during execution
– A variable may have different addresses at different places
in a program
– If two variable names can be used to access the same
memory location, they are called aliases
– Aliases are created via pointers, reference variables, C and
C++ unions
– Aliases are harmful to readability (program
readers must remember all of them)

Variables Attributes (continued)
• Type - determines the range of values of variables
and the set of operations that are defined for values
of that type; in the case of floating point, type also
determines the precision
• Value - the contents of the location with which the
variable is associated
- The l-value of a variable is its address
- The r-value of a variable is its value
• Abstract memory cell - the physical cell or collection of
cells associated with a variable

The Concept of Binding
A binding is an association between an entity
and an attribute, such as between a variable
and its type or value, or between an
operation and a symbol
• Binding time is the time at which a binding
takes place.

Possible Binding Times
• Language design time -- bind operator
symbols to operations
• Language implementation time-- bind
floating point type to a representation
• Compile time -- bind a variable to a type in
C or Java
• Load time -- bind a C or C++ static
variable to a memory cell)
• Runtime -- bind a nonstatic local variable to
a memory cell

Static and Dynamic Binding
• A binding is static if it first occurs before run
time and remains unchanged throughout
program execution.
• A binding is dynamic if it first occurs during
execution or can change during execution of
the program

Type Binding
• How is a type specified?
• When does the binding take place?
• If static, the type may be specified by either
an explicit or an implicit declaration

Explicit/Implicit Declaration
• An explicit declaration is a program statement
used for declaring the types of variables
• An implicit declaration is a default mechanism
for specifying types of variables through
default conventions, rather than declaration
statements
• Basic, Perl, Ruby, JavaScript, and PHP provide
implicit declarations
– Advantage: writability (a minor convenience)
– Disadvantage: reliability (less trouble with Perl)

Explicit/Implicit Declaration (continued)
• Some languages use type inferencing to
determine types of variables (context)
– C# - a variable can be declared with var and an
initial value. The initial value sets the type
– Visual Basic 9.0+, ML, Haskell, and F# use type
inferencing. The context of the appearance of a
variable determines its type

Dynamic Type Binding
• Dynamic Type Binding (JavaScript, Python,
Ruby, PHP, and C# (limited))
• Specified through an assignment statement
e.g., JavaScript
list = [2, 4.33, 6, 8];
list = 17.3;
– Advantage: flexibility (generic program units)
– Disadvantages:
• High cost (dynamic type checking and interpretation)
• Type error detection by the compiler is difficult

Variable Attributes (continued)
• Storage Bindings & Lifetime
– Allocation - getting a cell from some pool of
available cells
– Deallocation - putting a cell back into the pool
• The lifetime of a variable is the time during
which it is bound to a particular memory cell

Categories of Variables by Lifetimes
• Static--bound to memory cells before
execution begins and remains bound to the
same memory cell throughout execution,
e.g., C and C++ static variables in functions
– Advantages: efficiency (direct addressing),
history-sensitive subprogram support
– Disadvantage: lack of flexibility (no recursion)

• Stack-dynamic--Storage bindings are created for
variables when their declaration statements are
elaborated.
(A declaration is elaborated when the executable
code associated with it is executed)
• If scalar, all attributes except address are statically
bound
– local variables in C subprograms (not declared static) and
Java methods
• Advantage: allows recursion; conserves storage
• Disadvantages:
– Overhead of allocation and deallocation
– Subprograms cannot be history sensitive
– Inefficient references (indirect addressing)

• Explicit heap-dynamic -- Allocated and deallocated by
explicit directives, specified by the programmer,
which take effect during execution
• Referenced only through pointers or references, e.g.
dynamic objects in C++ (via new and delete), all
objects in Java
• Advantage: provides for dynamic storage
management
• Disadvantage: inefficient and unreliable

• Implicit heap-dynamic--Allocation and
deallocation caused by assignment
statements
– all variables in APL; all strings and arrays in Perl,
JavaScript, and PHP
• Advantage: flexibility (generic code)
• Disadvantages:
– Inefficient, because all attributes are dynamic
– Loss of error detection

Variable Attributes: Scope
• The scope of a variable is the range of statements
over which it is visible
• The local variables of a program unit are those that
are declared in that unit
• The nonlocal variables of a program unit are those
that are visible in the unit but not declared there
• Global variables are a special category of nonlocal
variables
• The scope rules of a language determine how
references to names are associated with variables

Static Scope
• Based on program text
• To connect a name reference to a variable, you (or the
compiler) must find the declaration
• Search process: search declarations, first locally, then in
increasingly larger enclosing scopes, until one is found
for the given name
• Enclosing static scopes (to a specific scope) are called
its static ancestors; the nearest static ancestor is called
a static parent
• Some languages allow nested subprogram definitions,
which create nested static scopes (e.g., Ada, JavaScript,
Common Lisp, Scheme, Fortran 2003+, F#, and Python)

Scope (continued)
• Variables can be hidden from a unit by
having a "closer" variable with the same
name
• Ada allows access to “hidden” vars

Blocks
– A method of creating static scopes inside program units--
from ALGOL 60
– Example in C:
void sub() {
int count;
while (...) {
int count;
count++;
...
}
…
}
- Note: legal in C and C++, but not in Java
and C# - too error-prone

Declaration Order
• C99, C++, Java, and C# allow variable
declarations to appear anywhere a statement
can appear
– In C99, C++, and Java, the scope of all local
variables is from the declaration to the end of the
block
– In the official documentation of C#, the scope of
any variable declared in a block is the whole block,
regardless of the position of the declaration in the
block
• However, that is misleading, because a variable still
must be declared before it can be used

Declaration Order (continued)
• In C++, Java, and C#, variables can be
declared in for statements
– The scope of such variables is restricted to the for
construct

Global Scope
• C, C++, PHP, and Python support a program
structure that consists of a sequence of
function definitions in a file
– These languages allow variable declarations to
appear outside function definitions
• C and C++have both declarations (just
attributes) and definitions (attributes and
storage)
– A declaration outside a function definition specifies
that it is defined in another file

Global Scope (continued)
• PHP
– Programs are embedded in HTML markup
documents, in any number of fragments, some
statements and some function definitions
– The scope of a variable (implicitly) declared in a
function is local to the function
– The scope of a variable implicitly declared outside
functions is from the declaration to the end of the
program, but skips over any intervening functions
• Global variables can be accessed in a function through
the $GLOBALS array or by declaring it global

Global Scope (continued)
• Python
– A global variable can be referenced in functions,
but can be assigned in a function only if it has
been declared to be global in the function

Evaluation of Static Scoping
• Works well in many situations
• Problems:
– In most cases, too much access is possible
– As a program evolves, the initial structure is
destroyed and local variables often become
global; subprograms also gravitate toward
become global, rather than nested

Dynamic Scope
• Based on calling sequences of program units,
not their textual layout (temporal versus
spatial)
• References to variables are connected to
declarations by searching back through the
chain of subprogram calls that forced
execution to this point

Scope Example
• Evaluation of Dynamic Scoping:
– Advantage: convenience
– Disadvantages:
1. While a subprogram is executing, its variables are
visible to all subprograms it calls
2. Impossible to statically type check
3. Poor readability- it is not possible to statically
determine the type of a variable

Scope and Lifetime
• Scope and lifetime are sometimes closely
related, but are different concepts
• Consider a static variable in a C or C++
function

Referencing Environments
• The referencing environment of a statement is the
collection of all names that are visible in the
statement
• In a static-scoped language, it is the local variables
plus all of the visible variables in all of the enclosing
scopes
• A subprogram is active if its execution has begun but
has not yet terminated
• In a dynamic-scoped language, the referencing
environment is the local variables plus all visible
variables in all active subprograms

Named Constants
• A named constant is a variable that is bound to a
value only when it is bound to storage
• Advantages: readability and modifiability
• Used to parameterize programs
• The binding of values to named constants can be
either static (called manifest constants) or dynamic
• Languages:
– C++ and Java: expressions of any kind, dynamically bound
– C# has two kinds, readonly and const
- the values of const named constants are bound at
compile time
- The values of readonly named constants are
dynamically bound

Summary
• Case sensitivity and the relationship of names to
special words represent design issues of names
• Variables are characterized by the sextuples: name,
address, value, type, lifetime, scope
• Binding is the association of attributes with program
entities
• Scalar variables are categorized as: static, stack
dynamic, explicit heap dynamic, implicit heap
dynamic
• Strong typing means detecting all type errors

Theory of programming language chapter 5

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