Names, Bindings, Type Checking and Scopes

SE 2002
PROGRAMMING LANGUAGES
Assist. Prof. Dr. M. Alper TUNGA
(alper.tunga@bahcesehir.edu.tr)
(http://akademik.bahcesehir.edu.tr/~alper)

CHAPTER 3
NAMES, BINDINGS,
TYPE CHECKING and SCOPES

INTRODUCTION
▶ Imperative languages are abstractions of von
Neumann architecture
◆ Memory
◆ Processor
▶ The abstractions in a language for the memory
cells of the machine are variables.
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NAMES
▶ A name is a string of characters used to identify
some entity in a program.
▶ Primary design issues for names:
◆ Are names case sensitive
◆ Are special words reserved words or keywords
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NAMES
▶ Length
◆ If too short, they cannot be connotative
◆ Language examples:
✔ FORTRAN I: maximum 6
✔ COBOL: maximum 30
✔ FORTRAN 90 and C89: maximum 31
✔ C99: maximum 63
✔ C#, Ada, and Java: no limit, and all are significant
✔ C++: no limit, but implementers often impose one
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NAMES
▶ Legal names
◆ Names in most programming languages have the
same form:
✔ a letter followed by a string consisting of letters,
digits, and underscore characters (_).
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NAMES
▶ Case sensitivity
◆ Names in the C-based languages are case sensitive
◆ Names in others are not
◆ Disadvantage: readability (names that look alike are
different)
✔ Worse in C++, Java, and C# because predefined
names are mixed case
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NAMES
▶ Special words
◆ Special words in programming languages are used
to make programs more readable by naming actions
to be performed.
◆ Used to separate the syntactic entities of programs
◆ In most languages, special words are classified as
reserved words, but in some they are only
keywords.
✔ 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
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VARIABLES
▶ A variable is an abstraction of a computer
memory cell or collection of cells.
▶ Variables can be characterized as a sextuple of
attributes:
◆ Name
◆ Address
◆ Value
◆ Type
◆ Lifetime
◆ Scope
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VARIABLES
▶ Name
◆ Most variables have names.
◆ Names in most programming languages have the
same form:
✔ a letter followed by a string consisting of letters,
digits, and underscore characters (_).
▶ Address
◆ The address of a variable is the machine memory
address with which it is associated
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VARIABLES
▶ Type
◆ The type of a variable 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
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VARIABLES
▶ Value
◆ The value of a variable is the contents of the
location (memory cell) with which the variable is
associated.
◆ Abstract memory cell - the physical cell or collection
of cells associated with a variable
◆ The l-value of a variable is its address
◆ The r-value of a variable is its value
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THE CONCEPT OF BINDING
▶ A binding is an association, such as between an
attribute and an entity, or between an operation
and a symbol
▶ Binding time is the time at which a binding takes
place.
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THE CONCEPT OF BINDING
▶ Possible Binding Times
◆ Language design time
◆ Language implementation time
◆ Compile time
◆ Load time
◆ Link time or run time
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THE CONCEPT OF BINDING
▶ 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.
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THE CONCEPT OF BINDING
▶ Type Binding
◆ Before a variable can be referenced in a program, it
must be bound to a data type.
◆ The two aspects of this binding are
✔ how the type is specified,
✔ when the binding takes place.
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THE CONCEPT OF BINDING
▶ Static Type Binding
◆ Types can be specified statically through some form
of explicit or 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 (the first appearance of
the variable in the program)
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THE CONCEPT OF BINDING
▶ Dynamic Type Binding
◆ With dynamic binding, the type of a variable is not
specified by a decIaration statement, nor can it be
determined by the spelling of its name.
◆ Instead, the variable is bound to a type when it is
assigned a value in an assignment statement.
◆ When the assignment statement is executed, the
variable being assigned is bound to the type of the
value of the expression on the right side of the
assignment.
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THE CONCEPT OF BINDING
▶ Type Inference
◆ Rather than by assignment statement, types are
determined (by the compiler) from the context of
the reference
◆ Type inference is used in ML, Miranda, and Haskell
languages.
◆ Example: Consider the following ML function.
✔ fun square(x)=x*x;
»
* is an arithmetic operator and the default numeric type
is int.
✔ fun circumf(r)=3.14159*r*r;
»
takes a floating-point argument and produces a floatingpoint
result.
✔ fun square(x):real=x*x;
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THE CONCEPT OF BINDING
▶ Storage Bindings & Lifetime
◆ Allocation - getting a cell, to which a variable can
be bound, 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
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THE CONCEPT OF BINDING
▶ Storage Bindings & Lifetime (cont.)
◆ To investigate storage bindings of variables, it is
convenient to separate scalar variables into four
categories, according to their lifetimes:
✔ static
✔ stack–dynamic
✔ explicit heap–dynamic
✔ implicit heap–dynamic
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THE CONCEPT OF BINDING
▶ Storage Bindings & Lifetime (cont.)
◆ Static Variables
✔ Static variables are those that are bound to memory
cells before program execution begins and remain
bound to those same memory cells until program
execution terminates.
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THE CONCEPT OF BINDING
▶ Storage Bindings & Lifetime (cont.)
◆ Stack–Dynamic Variables
✔ Stack-dynamic variables are those whose storage
bindings are created when their declaration
statements are elaborated.
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THE CONCEPT OF BINDING
▶ Storage Bindings & Lifetime (cont.)
◆ Explicit heap–dynamic variables
✔ These variables are nameless (abstract) memory cells
that are allocated and deallocated by explicit
directives, specified by the programmer, which take
effect during execution.
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THE CONCEPT OF BINDING
▶ Storage Bindings & Lifetime (cont.)
◆ Implicit heap–dynamic variables
✔ These variables are bound to heap storage only when
they are assigned values.
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TYPE CHECKING
▶ Type checking is the activity of ensuring that the
operands of an operator are of compatible types
◆ A compatible type is one that is either legal for the
operator, or is allowed under language rules to be
implicitly converted, by compiler- generated code,
to a legal type.
✔ This automatic conversion is called a coercion.
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TYPE CHECKING
▶ A type error is the application of an operator to
an operand of an inappropriate type.
▶ A programming language is strongly typed if
type errors are always detected.
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TYPE CHECKING
▶ If all type bindings are static, nearly all type
checking can be static.
▶ If type bindings are dynamic, type checking must
be dynamic.
◆ Requires type checking at run time
◆ Javascript and PHP
▶ It is better to detect errors at compile time than at
run time, because the earlier correction is usually
less costly.
▶ The penalty for static checking is reduced
programmer flexibility.
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STRONG TYPING
▶ A programming language is strongly typed if
type errors are always detected.
▶ This requires that the types of all operands can be
determined, either at compile time or at run time.
▶ The importance of strong typing lies in its ability to
detect all misuses of variables that result in type
errors.
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SCOPE
▶ The scope of a variable is the range of statements
over which it is visible.
▶ A variable is visible in a statement if it can be
referenced in that statement.
▶ The nonlocal variables of a program unit are
those that are visible within the program unit but
are not declared there.
▶ The scope rules of a language determine how
references to names are associated with variables
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SCOPE
▶ Static Scope
◆ The method of binding names to nonlocal variables.
◆ To connect a name reference to a variable, you (or
the compiler) must find the declaration
✔ This permits a human program reader to determine
the type of every variable in the program.
◆ 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
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SCOPE
▶ Dynamic Scope
◆ Based on calling sequences of program units, not
their textual layout (temporal versus spatial)
✔ Thus the scope can be determined only at run time.
◆ References to variables are connected to
declarations by searching back through the chain of
subprogram calls that forced execution to this point
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