Simula is the name of two
simulation programming languages, Simula I and Simula 67, developed in the 1960s at the
Norwegian Computing Center in
Oslo
Oslo ( or ; ) is the capital and most populous city of Norway. It constitutes both a county and a municipality. The municipality of Oslo had a population of in 2022, while the city's greater urban area had a population of 1,064,235 in 2022 ...
, by
Ole-Johan Dahl and
Kristen Nygaard.
Syntactically, it is an approximate
superset of
ALGOL 60
ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a ...
,
and was also influenced by the design of
SIMSCRIPT.
Simula 67 introduced
objects,
classes,
inheritance
Inheritance is the practice of receiving private property, titles, debts, entitlements, privileges, rights, and obligations upon the death of an individual. The rules of inheritance differ among societies and have changed over time. Offi ...
and
subclasses,
virtual procedures,
coroutines,
and
discrete event simulation,
and featured
garbage collection.
Other forms of
subtyping (besides inheriting subclasses) were introduced in Simula derivatives.
Simula is considered the
first object-oriented programming language. As its name suggests, the first Simula version by 1962 was designed for doing
simulations; Simula 67 though was designed to be a general-purpose
programming language
A programming language is a system of notation for writing computer programs.
Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...
and provided the framework for many of the features of object-oriented languages today.
Simula has been used in a wide range of applications such as simulating
very-large-scale integration (VLSI) designs,
process modeling,
communication protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...
s,
algorithm
In mathematics and computer science, an algorithm () is a finite sequence of Rigour#Mathematics, mathematically rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algo ...
s, and other applications such as
typesetting,
computer graphics
Computer graphics deals with generating images and art with the aid of computers. Computer graphics is a core technology in digital photography, film, video games, digital art, cell phone and computer displays, and many specialized applications. ...
, and
education
Education is the transmission of knowledge and skills and the development of character traits. Formal education occurs within a structured institutional framework, such as public schools, following a curriculum. Non-formal education als ...
.
Computer scientists such as
Bjarne Stroustrup, creator of C++, and
James Gosling, creator of Java, have acknowledged Simula as a major influence. Simula-type objects are reimplemented in
C++,
Object Pascal,
Java
Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
,
C#, and many other languages.
History
The following account is based on Jan Rune Holmevik's historical essay.
Kristen Nygaard started writing computer simulation programs in 1957. Nygaard saw a need for a better way to describe the heterogeneity and the
operation of a system. To further develop his ideas for a
computer language
A computer language is a formal language used to communicate with a computer. Types of computer languages include:
* Software construction#Construction languages, Construction language – all forms of communication by which a human can Comput ...
to describe a system, Nygaard realized that he needed someone with more
computer programming
Computer programming or coding is the composition of sequences of instructions, called computer program, programs, that computers can follow to perform tasks. It involves designing and implementing algorithms, step-by-step specifications of proc ...
skills than he had.
Ole-Johan Dahl joined him on his work in January 1962. Shortly after, the decision was made to link the language to
ALGOL 60
ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a ...
. By May 1962, the main concepts for a
simulation language were established; ''SIMULA I'', a specialized programming language designed for simulating discrete event systems, was born.
Kristen Nygaard was invited to visit the
Eckert–Mauchly Computer Corporation
The Eckert–Mauchly Computer Corporation (EMCC) (March 1946 – 1950) was a computer company founded by J. Presper Eckert and John Mauchly. It was incorporated on December 22, 1947. After building the ENIAC at the University of Penns ...
in late May 1962 in connection with the marketing of their new
UNIVAC 1107 computer. At that visit, Nygaard presented the ideas of Simula to
Robert Bemer, the director of systems programming at
Univac
UNIVAC (Universal Automatic Computer) was a line of electronic digital stored-program computers starting with the products of the Eckert–Mauchly Computer Corporation. Later the name was applied to a division of the Remington Rand company and ...
. Bemer was a great
ALGOL fan and found the Simula project compelling. Bemer was also
chairperson of a session at the second international conference on information processing hosted by
International Federation for Information Processing (IFIP). He invited Nygaard, who presented the paper "SIMULA – An Extension of ALGOL to the Description of Discrete-Event Networks".
The
Norwegian Computing Center got a
UNIVAC 1107 in August 1963 at a considerable discount, on which Dahl implemented the SIMULA I under contract with UNIVAC. The implementation was based on the UNIVAC
ALGOL 60
ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a ...
compiler. SIMULA I was fully operational on the UNIVAC 1107 by January 1965. In the following few years, Dahl and Nygaard spent a lot of time teaching Simula. Simula spread to several countries around the world and SIMULA I was later implemented on other computers including the
Burroughs B5500 and the Russian
Ural-16.
In 1966
C. A. R. Hoare introduced the concept of record class construct, which Dahl and Nygaard extended with the concept of prefixing and other features to meet their requirements for a generalized process concept. Dahl and Nygaard presented their paper on
class and
subclass declarations at the IFIP Working Conference on
simulation languages in
Oslo
Oslo ( or ; ) is the capital and most populous city of Norway. It constitutes both a county and a municipality. The municipality of Oslo had a population of in 2022, while the city's greater urban area had a population of 1,064,235 in 2022 ...
, May 1967. This paper became the first formal definition of Simula 67. In June 1967, a conference was held to standardize the language and initiate a number of implementations. Dahl proposed to unify the
type and the class concept. This led to serious discussions, and the proposal was rejected by the board. Simula 67 was formally standardized on the first meeting of the Simula Standards Group (SSG) in February 1968.
Simula was influential in the development of
Smalltalk and later
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...
languages. It also helped inspire the
actor model of concurrent computation although Simula only supports
coroutines and not true
concurrency.
In the late sixties and the early seventies, there were four main implementations of Simula:
*
UNIVAC
UNIVAC (Universal Automatic Computer) was a line of electronic digital stored-program computers starting with the products of the Eckert–Mauchly Computer Corporation. Later the name was applied to a division of the Remington Rand company and ...
1100 by
Norwegian Computing Center (NCC)
*
System/360 and
System/370 by NCC
*
CDC 3000 by
University of Oslo
The University of Oslo (; ) is a public university, public research university located in Oslo, Norway. It is the List of oldest universities in continuous operation#Europe, oldest university in Norway. Originally named the Royal Frederick Univ ...
's Joint Computer Installation at Kjeller
*
TOPS-10 by
Swedish National Defence Research Institute (FOA)
These implementations were ported to a wide range of platforms. The
TOPS-10 implemented the concept of public, protected, and private member variables and procedures, that later was integrated into Simula Standard in 1986.
Simula Standard 1986 is the latest standard and is ported to a wide range of platforms. There are mainly four implementations:
* Simula AS
* Lund Simula
* GNU Cim
* Portable Simula Revisited
In November 2001, Dahl and Nygaard were awarded the
IEEE John von Neumann Medal by the
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers (IEEE) is an American 501(c)(3) public charity professional organization for electrical engineering, electronics engineering, and other related disciplines.
The IEEE has a corporate office ...
"For the introduction of the concepts underlying object-oriented programming through the design and implementation of SIMULA 67". In April 2002, they received the 2001 A. M.
Turing Award by the
Association for Computing Machinery (ACM), with the citation: "For ideas fundamental to the emergence of object oriented programming, through their design of the programming languages Simula I and Simula 67." Dahl and Nygaard died in June and August of that year, respectively, before the ACM Turing Award Lecture
that was scheduled to be delivered at the November 2002
OOPSLA conference in Seattle.
Simula Research Laboratory is a
research institute
A research institute, research centre, or research organization is an establishment founded for doing research. Research institutes may specialize in basic research or may be oriented to applied research. Although the term often implies natural ...
named after the Simula language, and Nygaard held a part-time position there from the opening in 2001. The new Computer Science building at the
University of Oslo
The University of Oslo (; ) is a public university, public research university located in Oslo, Norway. It is the List of oldest universities in continuous operation#Europe, oldest university in Norway. Originally named the Royal Frederick Univ ...
is named Ole Johan Dahl's House, in Dahl's honour, and the main auditorium is named Simula.
Sample code
Minimal program
The empty
computer file is the minimal
program in Simula, measured by the size of the
source code. It consists of one thing only; a dummy
statement.
However, the minimal program is more conveniently represented as an empty block:
Begin
End;
It begins executing and immediately terminates. The language lacks any
return value from the program.
Classic Hello world
An example of a
Hello world program in Simula:
Begin
OutText ("Hello, World!");
Outimage;
End;
Simula is
case-insensitive.
Classes, subclasses and virtual procedures
A more realistic example with use of classes,
subclasses
and virtual procedures:
Begin
Class Glyph;
Virtual: Procedure print Is Procedure print;;
Begin
End;
Glyph Class Char (c);
Character c;
Begin
Procedure print;
OutChar(c);
End;
Glyph Class Line (elements);
Ref (Glyph) Array elements;
Begin
Procedure print;
Begin
Integer i;
For i:= 1 Step 1 Until UpperBound (elements, 1) Do
elements (i).print;
OutImage;
End;
End;
Ref (Glyph) rg;
Ref (Glyph) Array rgs (1 : 4);
''! Main program;''
rgs (1):- New Char ('A');
rgs (2):- New Char ('b');
rgs (3):- New Char ('b');
rgs (4):- New Char ('a');
rg:- New Line (rgs);
rg.print;
End;
The above example has one
super class (Glyph) with two
subclasses (
Char
and
Line
). There is one
virtual procedure with two
implementations. The execution starts by executing the main program. Simula lacks the concept of
abstract classes, since classes with pure
virtual procedures can be
instantiated. This means that in the above example, all classes can be instantiated. Calling a pure virtual procedure will however produce a
run-time error.
Call by name
Simula supports
call by name so the
Jensen's Device can easily be implemented. However, the default transmission mode for simple parameter is
call by value, contrary to
ALGOL which used call by name. The source code for the Jensen's Device must therefore specify call by name for the parameters when compiled by a Simula compiler.
Another much simpler example is the
summation function which can be implemented as follows:
Real Procedure Sigma (k, m, n, u);
Name k, u;
Integer k, m, n; Real u;
Begin
Real s;
k:= m;
While k <= n Do Begin s:= s + u; k:= k + 1; End;
Sigma:= s;
End;
The above code uses call by name for the controlling variable (
k
) and the expression (
u
).
This allows the controlling variable to be used in the expression.
Note that the Simula standard allows for certain restrictions on the controlling variable
in a
for loop
In computer science, a for-loop or for loop is a control flow Statement (computer science), statement for specifying iteration. Specifically, a for-loop functions by running a section of code repeatedly until a certain condition has been satisfi ...
. The above code therefore uses a while loop for maximum portability.
The following:
can then be implemented as follows:
Z:= Sigma (i, 1, 100, 1 / (i + a) ** 2);
Simulation
Simula includes a
simulation package for doing
discrete event simulations. This simulation package is based on Simula's object-oriented features and its
coroutine concept.
Sam, Sally, and Andy are shopping for clothes. They must share one fitting room. Each one of them is browsing the store for about 12 minutes and then uses the fitting room exclusively for about three minutes, each following a normal distribution. A simulation of their fitting room experience is as follows:
Simulation Begin
Class FittingRoom; Begin
Ref (Head) door;
Boolean inUse;
Procedure request; Begin
If inUse Then Begin
Wait (door);
door.First.Out;
End;
inUse:= True;
End;
Procedure leave; Begin
inUse:= False;
Activate door.First;
End;
door:- New Head;
End;
Procedure report (message); Text message; Begin
OutFix (Time, 2, 0); OutText (": " & message); OutImage;
End;
Process Class Person (pname); Text pname; Begin
While True Do Begin
Hold (Normal (12, 4, u));
report (pname & " is requesting the fitting room");
fittingroom1.request;
report (pname & " has entered the fitting room");
Hold (Normal (3, 1, u));
fittingroom1.leave;
report (pname & " has left the fitting room");
End;
End;
Integer u;
Ref (FittingRoom) fittingRoom1;
fittingRoom1:- New FittingRoom;
Activate New Person ("Sam");
Activate New Person ("Sally");
Activate New Person ("Andy");
Hold (100);
End;
The main block is prefixed with
Simulation
for enabling simulation. The simulation package can be used on any block and simulations can even be nested when simulating someone doing simulations.
The fitting room object uses a queue (
door
) for getting access to the fitting room. When someone requests the fitting room and it's in use they must wait in this queue (
Wait (door)
). When someone leaves the fitting room the first one (if any) is released from the queue (
Activate door.first
) and accordingly removed from the door queue (
door.First.Out
).
Person is a subclass of
Process
and its activity is described using hold (time for browsing the store and time spent in the fitting room) and calls procedures in the fitting room object for requesting and leaving the fitting room.
The main program creates all the objects and activates all the person objects to put them into the event queue. The main program holds for 100 minutes of simulated time before the program terminates.
Notes
Sources
*
Further reading
*
External links
(last working version at archive.org, accessed 2022-02-26)*
{{Authority control
ALGOL 60 dialect
Class-based programming languages
Norwegian inventions
Programming languages created in 1962
Science and technology in Norway
Simulation programming languages
Programming languages
Statically typed programming languages