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

• 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

sourcecode

input interpreter output

Compilation

executablecode

processorinput output

sourcecode

compile targetcode

furthertranslation

executablecode

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