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CHAPTER THREE
Type Systems
and Semantics
Programming Languages – Principles and Paradigms
by Allen Tucker, Robert Noonan
Type Systems :
A Type is a well-defined set of values and
operations.
In C - Int, float, char.
In FORTRAN – INTEGER, REAL, COMPLEX.
A Type System associates types with variables
and other objects defined in a program.
A Type Error occurs when an operation is
attempted on a value for which it is not defined
A Strongly Typed language allows all type errors
to be detected before the offending statement
can be executed.
Static Typing :
A Statically Typed language associates a single
type with a variable for the entire existence of
that variable.
Variable types are determined at compile time.
This means that type errors can also be
checked at compile time.
Run time checking can also be reduced since
the need to coerce value types can be detected
at compile time.
Examples – C, C++, Java.
Dynamic Typing :
A Dynamically Typed language allows the a
variable’s type to change during program
execution.
This can be very convenient but can make
debugging more difficult.
There can be performance penalties (why?)
Examples – Lisp, Scheme, Perl.
Formalizing Typing :
BNF is not capable of defining type checking
requirements for languages.
Can’t ensure all variables have unique names.
Can’t express the need to declare all variables prior
to use.
A program is Type-safe if it is free of typing
errors.
A Type Map is used to formally define the rules
for writing type-safe programs.
Contains pairs of declared variables and their types.
Analogous to a symbol table.
Formalizing Typing :
Programs written in strongly typed languages
are type safe.
If all the programs that can be written in a
language are type safe then the language is
strongly typed.
Dynamically typed languages are also type safe
as long as they can always coerce values at
runtime (otherwise?)
Programs written in strongly typed languages
are defined as having two parts:
A declarations section.
A body section.
Type Checking in jay :
Each declared Variable must have a unique
Identifier.
Each Variable’s type must be either int or
boolean.
Each Variable referenced within any Expression
in a program must have been declared.
For every Assignment statement the type of the
target variable must agree with the type of the
source expression.
For every Conditional and Loop the expression
type must be boolean.
Type Checking in jay :
Expression result types are determined as follows:
If the Expression is a Variable or Value then the result
is of the same type as the Variable or Value.
If the Expression’s Operator is arithmetic (+, -, *, /)
then…
All terms must be of type int.
The result is of type int.
If the Operator is relational (<, <=, >, >=, ==, !=) then…
All terms must be of type int.
The result is of type boolean.
If the Operator is boolean (&&, ||, !) then…
All terms must be of type boolean.
The result is of type boolean.
A C/C++ Factorial Function
Figure 3.2
Next time…
More On
Semantics
Programming Paradigms:
Imperative Programming.
Programs are a series of steps with input and output.
Traditional stuff like C, Fortran, and Cobol.
Object Oriented Programming (OOP).
Programs consist of objects passing messages.
Smalltalk, C++, and Java.
Functional Programming.
Programs are collections of functions.
Lisp, Scheme, Haskell, and ML.
Programming Paradigms:
Logic (Declarative) Programming.
Programs are collections of logical declarations.
Prolog and expert systems.
Event-driven Programming.
Programs are loops responding to events.
Visual Basic, Java, but can be almost any language.
Concurrent Programming.
Programs consist of cooperating processes.
High Performance Fortran (HPF), Linda, and SR.
Choose One!