Download Introduction - Valdosta State University

Theory of Programming
Languages
Introduction
What is a Programming
Language?
• John von Neumann (1940’s)
– Stored program concept
– CPU actions determined by “codes” stored in memory
• Assembly Languages
– Mnemonics for the Instruction Codes
– Low-level
• One instruction per operation
– Machine-dependent
High-Level Languages
• Code abstractions allowed more readable
and writeable programs
– Assignments
– Loops
– Conditionals
• Machine – independent
• Retains the processor-model of computation
• Allows human to human communication
Definition
• A Programming Language is a notational
system for describing computation in
machine-readable and human-readable form
Computation
• Turing machine
– Mathematical concept of a machine whose
operation is simple enough to be described with
great precision
– Powerful enough to to perform any
computation a computer can
– Church’s Theorem: It is impossible to build a
machine that is inherently more powerful than a
Turing machine.
Computation
• (For our purposes) Any process which can
be carried out by a computer
• We will concentrate on general-purpose
languages that are designed to be used in
general processing
Machine Readablilty
• Simple enough to allow efficient translation
– There must be an algorithm to translate the
language
– This algorithm must not have too great a
complexity
– This is usually ensured by restricting the
structure of the programming language to that
of Context-free grammars (CFG’s)
Human Readability
• The language should allow abstractions of
actions of the computer that are easy to
understand
– It should tend to resemble a natural language
(English for us)
• For large programs, there must be suitable
mechanisms for reducing the amount of
detail required to understand the program as
a whole
Abstractions in Programming
Languages
• Data Abstraction
– Abstract properties of the data
• Strings
• Trees
• Control Abstraction
– Abstract properties of the transfer of control
• Loops
• Procedure calls (sort, search)
Data Abstractions
• Basic Abstractions
– Abstract the internal representation of data values
• Variables and data types
• Structured Abstractions
– Arrays and Records
– Data Structures
• Unit Abstractions
– Data encapsulation
– Information hiding
Control Abstractions
• Basic Abstractions
– Variable assignment
– GOTO’s
• Structured Abstractions
– Selection
– Looping
– Sub programs
Unit Abstractions
• Grouping a collection of related procedures
into a unit whose inner workings do not
need to be understood to understand the
program
– Ex: all I/O procedures
– Ex: all statistical procedures (mean, mode,
median, variance, etc)
Other Abstractions
• Parallel processing mechanisms
– Threads
– Processes
– Tasks (Ada)
Computational Paradigms
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Imperative (Procedural) Programming
Object-Oriented Programming
Functional Programming
Logic Programming
Event-driven Programming
Concurrent Programming
Imperative Programming
• Imperative programming is characterized by
– Sequential execution of instructions
– Use of variables representing memory locations
– Use of assignment to change values of variables
Object-oriented Programming
• Three major topics of object-oriented
programming
– Polymorphism
– Inheritance
– Encapsulation
• Object
– State
– Behavior
Functional Programming
• Functional programming bases computation
on evaluation of functions (or function
applications)
• No notion of variables or assignment
• Repetitive operations are done by recursive
functions (no notion of loops)
Logic Programming
• Logic programming is based on symbolic
logic
• A logic program is a collection of
declarations which are true about the
desired result
• No notion of flow-of-control
Event-driven Programming
• Programming based on response to events
• The underlying system recognizes the
events so there is no need for control-flow
mechanisms except in response to events
Concurrent Programming
• Synchronization
– Threads, for example
Language Definition
• Syntax
– The structure of the language (grammar)
– Notational systems for formal description
• BNF
• Syntax diagrams
• Semantics
– The meaning of the language constructs
– Notational systems for formal description
• Operational semantics
• Denotational semantics
• Axiomatic semantics
Language Translation
• For a programming language to be useful, it
needs a translator
– Interpreter
• A translator that executes a program directly
– Compiler
• A translator that produces an equivalent program in
a form suitable for execution
Interpretation
source
code
input
interpreter
output
Compilation
source
code
compile
target
code
further
translation
executable
code
executable
code
input
processor
output
Phases of translation
• Lexical analyzer (scanner)
• Syntax analyzer (parser)
• Semantic analyzer
Language Design
• Machine and human readability are
overriding requirements
• Facilities for the natural expression of the
structure of data
• Goal of abstraction is complexity control