01 introduction

Nature of Programming Languages
Introduction

Nguyễn Hữu Đức

Faculty of Information Technology
Hanoi University of Technology

Introduction What is a Programming Language? Abstractions in Programming Languages Computational Paradigms Language Definition La

Contents I

1 Introduction

2 What is a Programming Language?

3 Abstractions in Programming Languages

4 Computational Paradigms

5 Language Definition

6 Language Translation

7 Language Design

Nature of programming Languages N.H. Đức FACULTY OF INFORMATION TECHNOLOGY 2 / 54


Goal of this course

There are over 2500 programming languages
We shall not study any special programming language
We are going to study major principles and concepts of programming
languages.



Computers and Programming Languages

Before 1940s: “hard-wired” computers
“Special purpose” computing
After 1940s: John Von Neumann’s computers
series of codes could be stored as data and would be executed by CPU
“General purpose” computing



John Von Neumann’s Machine

developed in 1940’s so that a series of codes could be stored in memory
as data and would be executed by CPU.
“memory” : where programs and data are stored
“CPU” : sequentially executes instructions



What is a Programming Language?

binary instructions
low-level languages (i.e. assembly)
machine-dependent
difficult to write and to understand programs
low-level of abstraction
high-level languages (i.e. C, Java,...)
higher-level of abstraction
code does not change (or little change) from machine to machine
easier to write and to understand



What is a Programming Language?

Definition
A programming language is a notational system for describing computation in
machine-readable and human-readable form.



Computation

Turing machine
A mathematical model of computation
High precision
Can carry out any computation that any current computer can carry out


Computation

Church’s thesis
It is not possible to build a machine more powerful than a Turing machine
Practical aspect of this course
including mathematical computation, text processing, information storage
and retrieval,...
concentrating on “general purpose” programming languages (“special
purpose” languages exist)



Machine readability

Machine-readable language can be translated efficially by a computer
into a form that can be executed by the target computer.
There must be an algorithm that translates the language
This step-by-step process is unambiguous and finite
The algorithm mus be able to translate in time proportional to the size of
the program
O(n) complexity
Machine readability is ensured by restricting the structure of a
programming language to that of “context-free languages”



Human readability

Require that a programming language provide abstractions of the actions
of computers that are easilly understood
By someone who is not familiar with the details of the underlying
computer
Programming languages should tend to resemble natural languages



Human readability

Require suitable mechanisms for reducing the amount of detail required
for understanding the whole program
A small change to one part of a program should not require major changes
to the entire program
Requires collection of local information in one place
Programming language is part of the software development environment
Many people involved in software development
We will not study PLs from software engineering point of view
We will focus on languages themselves



Two Categories of Abstraction

Data abstraction
abstract properties of data (strings, numbers, ...) which is the subject of
computation
Control abstraction
abstract properties of the transfer of control
Ex. loops, conditional statements, procedure calls



Three Levels of Abstraction

Basic abstractions
collect together the most localized machine information
Structured abstractions
collect more global information about the structure of the program
Unit abstraction
collect information about entire pieces of a program



Data Abstractions - Basic Abstraction

Abstract internal representation of the common data values
Ex. integers are often stored in computer using a two’s complement
representation
Ex. floating point values are stored using IEEE 754 standard
Locations in memory are abstracted by a name – a variable
Kind of data values is given a name – data type

Example
var x : integer; // PASCAL
int x; // C



Data Abstractions - Structured Abstraction

Data structures abstract collection of data that are related
Array
Record in Pascal or struct in C
datatype in ML

Example
int a[10]; // Array in C

struct Tree { // struct in C
struct Tree *left, *right;
void *data;
}

datatype ’a tree = // datatype in ML
Nil
| Node of {left : ’a tree, right : ’a tree, data : ’a}



Data Abstractions - Unit Abstraction

Collect related code into specific locations in the program
Data encapsulation and Information Hiding
Ex. modules in ML, classes in object-oriented languages, packages in Java
class is also considered as a structured abstraction
Reusability
Ability of reuse data abstration in different parts of a program
Save writing code from scratch
Components : Operationally complete pieces of programs
Containers : Data structures containing other user-defined data
Basis for language library
Many interface standards (COM, CORBA)



Control Abstractions - Basic Abstraction

Statements in a programming language
combine a few machine instruction into a more understandable abstract
statement
Ex. “x := x + 3” fetches, adds and stores values
Ex. “GOTO 10” abstracts the jump instruction



Control Abstractions - Structured Abstraction

Divide a program into groups of instructions that are nested within tests
that govern their execution
Selection statement (“if”, “case”)
They can be nested




Selection statement in C
if (x > 0) {
numSols = 2;
r1 = sqrt(x);
r2 = - r1;
} else {
numSols = 0;
}




Selection statement in ML
case numSols(x) of
0 => []
2 => [sqrt(x), - sqrt(x)]




Looping mechanisms: for, while, repeat ... until,
loop
Subroutines (procedures)
Allows programmer to consider sequence of actions as one action
Subroutines must be declared
Subroutines must be called (they have point of invocation or activation)
Formal parameters are defined in declaration
Arguments or actual parameters are passed when subroutine called
Information about subroutine must be saved at the point of invocation and
restored when subroutine finishes
Runtime environment or operational stack
A subroutine can return a value or does not need to return a value when
finished




Loop and subroutine in C
int gcd(int u, int v) {
int x = u;
int y = v;
int t;
while ( y != 0) {
t = y;
y = x % y
x = t;
}
return x;
}

Loop and subroutine in ML
fun gcd(x, 0) = x
| gcd(x, y) = gcd(y, x mod y)



Control Abstractions - Unit Abstraction

Procedures can be collected into a package or a program unit
Carefully controlled interfaces
Can be understood without knowing the details
Reusability and library buiding are the same as data unit abstraction



Parallel Programming Mechanism

Programming languages that are capable of executing parts of a program
simultaneously
They also have mechanism for synchronization and communication
among programs or parts of a program



Abstraction Mechanism

All abstractions are provided for human readability
They should also provide all mechanisms to do any computation that a
Turing machine can do

Turing complete
A programming language is Turing complete provided it has integer variables
and arithmetic and sequentially executes statements, which include asignment,
selection (if) and loop (while) statement.



Computational Paradigms

Most of programming languages are designed based on Von Neumann
model
Sequential execution of instructions
Variables represent memory locations
Asignment allows program to operate on these memory variables
Programming languages characterized by these properties are called
imperative (or procedural) programming languages
It is not neccessary for a programming language to be imperative
Von Neumann model restricts parallel computation, non-deterministic
computation, or computation that does not depend on order (Von
Neumann Bottleneck)
Computational paradigms
Imperative paradigm
Functional paradigm
Logic paradigm
Object-oriented paradigm



Computational Paradigms - Imperative Paradigm

Properties of imperative programming languages
Sequential execution of instructions
Variables represent memory locations
Asignment allows program to operate on these memory variables
Sequentially execution of statements
Also called “procedural” programming languages
Most PLs today based on this model
Deterministic computation
Can’t use this to describe parallel computation
Typical examples: FORTRAN, PASCAL, C



Imperative Paradigm

Example of gcd in C
int gcd(int u, int v) {
int x = u;
int y = v;
int t;
while ( y != 0) {
t = y;
y = x % y
x = t;
}
return x;
}



Computational Paradigms - Functional Paradigm

Based on description of computation on the evaluation of functions or
the application of functions to known values
Also called “applicative” languages
Functional call
Passing the values as parameters
Actual evaluation of function
Returning values
No notion of variable or assignment
Repetitive operation can be done with recursion instead of loops
Typical examples: LISP, SCHEME, ML, HASKEL



Functional Paradigm

Example of gcd in ML
fun gcd(x, 0) = x
| gcd(x, y) = gcd(y, x mod y)



Computational Paradigms - Logic Paradigm

Based on symbolic logic
Program is a set of statements that describe what is true about a desired
result
No loops or selections
Control is suplied by the underlying program
Also called “descritive” language
Typical example: PROLOG



Logic Paradigm

Example of gcd in PROLOG
gcd(U, V, U) :- V = 0
gcd(U, V, X) :- not(V = 0),
Y is U mod V,
gcd(V, Y, X)



Computational Paradigms - Object-oriented Paradigm

Based on notion of an object which can be loosely described as a
collection of memory locations together with all the operations that can
change the values of these memory locations
Objects are grouped into classes that represent all the objects with the
same properties
Object can be created as an instance of a class
Members represent memory locations
Methods represent operations on these memory locations
Provide data encapsulation and information hiding
Typical example: C++, JAVA



Object-oriented Paradigm

Example of gcd in JAVA
public class IntWithGcd {
public IntWithGcd(int val) {value = val;}
public int intValue() { retuen value; }
public int gcd(int v) {
int z = value;
int y = v;
while ( y != 0) {
int t = y;
y = z % y;
z = t;
}
return z;
}

private int value;
};



Language Definition

A complete, precise description is needed
To know precisely what a computation does
Even today, most PLs are described informally by the so-call “reference
manual”
Reference manual is not precise
With these definitions, it is possible to reason mathematically about
programs
formal verification, proof of behavior



Language Definition

Formal definition of a language leads to standadization
Language becomes independent of hardware platform, OS, ...
ANSI (American National Standards Institute) and ISO (International
Organization for Standardization) standards
definitions of many languages including PASCAL, FORTRAN, C, C++,
Ada, PROLOG



Language Definition

From practical point of view, formal definition needed
because question about program’s behavior may arise
to provide discipline during the design of the language



Language Definition

Two parts of language definition
Syntax : structure of the language
Semantics : Meaning of the language



Language Syntax

Language syntax is like the grammar of a natural language
Language syntax can be given in context free grammar form
<if-statement> ::= if (<expression>) <statement>
[else <statement>]
Language syntax can be given in form using special character and
formatting
if-statement ::= if (expression) statement [else statement]



Language Syntax

Lexical structure
Similar to spelling in a natural language
provide structure of words or token in the language
if, else are token
“+”, “<=” are token
“;”, “.” are token



Language Semantics

Semantics describes the meaning of the language
effect of execution of a piece of code
much more complex and difficult to describe precisely

C if-statement, Kernighan and Richie [1988]
An if-statement is executed by first evaluating its expression, which must have
arithmetic or pointer type, including all side effects, and if it compares
unequal to 0, the statement following the expression is executed. If there is an
else part, and the expression is 0, the statement following the “else” is
executed.

What’s happen if there is no else part?



Language Semantics

Usin formal methods to describe the language semantics
No generally accepted method
There are several methods
operational semantics
denotational semantics
axiomatic semantics



Language Translation

There must be a translator that accepts a program written in the
programming language and
either executes the program directly (interpreter)
or transforms into a form in which it can be executed (compiler)



Interpretation

Source code

input interpreter output

one-step process that simulates the underlying machine



Compilation

Source target
code code
further Executable
compile transtaltion code

Executable code

input processor output

two-step process that simulates the underlying machine
output of the compiler is called the target code
most common target language is assembler language
assembler program is then generated to object code of the underlying
machine and then linked with other object codes by a linker and loaded
into memory by a loader


Compilation

Pseudo-translater
translates a source program into an intermediate code
interpreter will execute the intermediate code
A language is different from a translator for that language
Some translators may or may not adhere closely to the language definition



Phases of a Translator

Lexical Analyzer (or scanner) converts source code (in text form) into
sequence of tokens representing keywords, identifiers, constants,...
Syntax Analyzer (or parser) determines structure of the sequence of
token
Semantic Analyzer must determine enough of the meaning of the
program to allow the execution of the target code



Operations Performed by a Translator

Translater must maintain a run-time environment which has suitable
memory for program data and record the progress of execution
Interpreter itself maintains a run-time environment
Compiler maintains run-time environment indirectly through target code
Language may require a pre-processor which is run to make the program
suitable for translation

Example
#define PI 3.14



Static and Dynamic Properties

Static propeties
Determined prior to execution
Static memory allocation: all variables located in a fixed possitions during
the execution
Dynamic propeties
Determined during the execution
Dynamic memory allocation
Static properties are useful for compilers



Static and Dynamic Properties

Language with large amount of static properties more suitable for
compilation
Language with large amount of dynamic properties more suitable for
interpretation
Imperative languages tend to be compiled
Functional and logic languages tend to be interpreted
Languages like C have both aspects



Efficiency

Interpreted language generally less efficient than compiled language
Compiler efficiency can be boosted by optimization
Interpreters are better than compilers if interactive input and output
required



Error-handling

Translators should try to fix errors or provide meaningful error messages
Translators should be able to find more errors after the first one caught
Errors can be caught by
scanner: use illegal characters
parser: syntax error (Ex. x = 2 +* 6)
semantic analyser: undefined variables, incompatible type
Semantic error can be found during the execution
Index out-of range
Divide by zero



Error-handling

Not all logic error can be found by the language translator (Ex. infinite
loops)
Language specification often specifies
What error must be caught at compile time
What error must be generated at run time
What error go undetected



Debugging

Translator must provide option for debugging, for interfacing with OS
and with IDE



Language Design

Machine readability and Human readability are main requirements
Data abstractions and control abstractions
Complexity control


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