Analysis of information sources in references of the Wikipedia article "Simula" in Chinese language version.
In SIMULA 67, a block instance is permitted to outlive its calling statement, and to remain in existence for as long as the program needs to refer to it. It may even outlive the block instance that called it into existence. As a consequence, it is no longer possible to administer storage allocation as a simple stack; a general garbage-collector, including a scan-mark operation, is required to detect and reclaim those areas of store (local workspace of block instances) which can no longer be referenced by the running program. The reason for accepting this extra complexity is that it permits a wider range of concepts to be conveniently expressed. In particular, it clarifies the relationship between data and the operations which may be performed upon it, in a way which is awkward or impossible in ALGOL 60.
A procedure which is capable of giving rise to block instances which survive its call will be known as a class; and the instances will be known as objects of that class. A class may be declared, with or without parameters, in exactly the same way as a procedure ……
useful properties from the standpoint of concept modelling. ……
⑷ Language element. A block is itself a statement, which is a syntactic category of the language. Furthermore, through the procedure mechanism, reference to a block may be dissociated from its defining text.
One of the most powerful mechanisms for program structuring in ALGOL 60 is the block and procedure concept. It has the following useful properties from the standpoint of concept modelling.
⑴ Duality. A block head and block tail together define an entity which has properties and performs actions. Furthermore the properties may include a data structure as well as associated operators (local procedures).
In ALGOL 60, the rules of the language have been carefully designed to ensure that the lifetimes of block instances are nested, in the sense that those instances that are latest activated are the first to go out of existence. It is this feature that permits an ALGOL 60 implementation to take advantage of a stack as a method of dynamic storage allocation and relinquishment. But it has the disadvantage that a program which creates a new block instance can never interact with it as an object which exists and has attributes, since it has disappeared by the time the calling program regains control. Thus the calling program can observe only the results of the actions of the procedures it calls. Consequently, the operational aspects of a block are overemphasised; and algorithms (for example, matrix multiplication) are the only concepts that can be modelled.
useful properties from the standpoint of concept modelling. ……
⑶ Class of instances. In ALGOL 60 a sharp distinction is made between a block, which is a piece of program text, and a dynamic block instance, which is (a component of) a computing process. An immediate and useful consequence is that a block may be identified with the class of its potential activations. (Strictly speaking a "block" in this context means either the outermost block or a block immediately enclosed by a dynamic block instance.) Through the recursion mechanism of ALGOL 60 different instances of the same block may co-exist in a computing process at the same time.
useful properties from the standpoint of concept modelling. ……
⑵ Decomposition. A block where only local quantities are referenced is a completely selfcontained program component, which will function as specified in any context. Through a procedure heading a block (procedure) instance may interact with a calling sequence. Procedures which reference or change non-local variables represent a partial decomposition of the total task, which is useful for direct interaction with the program environment.
In SIMULA, a coroutine is represented by an object of some class, co-operating by means of resume instructions with objects of the same or another class, which are named by means of reference variables. ……
Thus a main program may establish a coroutine relationship with an object that it has generated, using the call/detach mechanism instead of the more symmetric resume/resume mechanism. In this case, the generated object remains subordinate to the main program, and for this reason is sometimes known as a Semicoroutine. ……
LetXandYbe objects, generated by a "master program"M. Assume thatMissues acall (X), thereby invoking an "active phase" ofX, terminated by a detach operation inX; and then issues acall (Y), and so forth. In this wayMmay act as a "supervisor" sequencing a pattern of active phases ofX,Y, and other objects. Each object is a "slave", which responds with an active phase each time it is called for, whereas M has the responsibility to define the large scale pattern of the entire computation.
Alternatively the decision making may be "decentralised", allowing an object itself to determine its dynamic successor by a resume operation.
The operationresume (Y), executed byX, combines an exit out ofX(bydetach) and a subsequentcall (Y), thereby bypassingM. Obligation to return toMis transferred toY.
C++ was designed to provide Simula’s facilities for program organization together with C’s efficiency and flexibility for systems programming.
Java is a blue collar language. It’s not PhD thesis material but a language for a job. Java feels very familiar to many different programmers because I had a very strong tendency to prefer things that had been used a lot over things that just sounded like a good idea. …… It has an object-oriented flavor that derives from a number of languages—Simula, C/C++, Objective C, Cedar/Mesa, Modula, and Smalltalk.
A procedure deviates from a block in that ⑴ it has a name and ⑵ may be referred to at several different places in the program, and ⑶ that parameters may be given to it when invoked. A procedure shares the property of a block that it is impossible to establish a reference to it or to its interior.
The procedure "call" is not a part of the Common Base, but is a natural part of a SIMULA 67 Common Base implementation. ……This definition of call is tentative, since the problem is currently being studied by a Technical Committee under the SIMULA Standards Group.
I wound up in graduate school at the University of Utah in the Fall of 1966, ……. …… The documentation was incomprehensible. Supposedly, this was the Case-Western Reserve 1107 Algol – but it had been doctored to make a language called Simula; the documentation read like Norwegian transliterated into English, which in fact it was. There were uses of words like activity and process that didn’t seem to coincide with normal English usage. ……
The weirdest part was the storage allocator, which did not obey a stack discipline as was usual for Algol. …… What Simula was allocating were structures very much like the instances of Sketchpad. There wee descriptions that acted like masters and they could create instances, each of which was an independent entity. What Sketchpad called masters and instances, Simula called activities and processes. Moreover, Simula was a procedural language for controlling Sketchpad-like objects, thus having considerably more flexibility than constraints (though at some cost in elegance) ……. ……
For the first time I thought of the whole as the entire computer and wondered why anyone would want to divide it up into weaker things called data structures and procedures. Why not divide it up into little computers, as time sharing was starting to? But not in dozens. Why not thousands of them, each simulating a useful structure? ……
It is not too much of an exaggeration to say that most of my ideas from then on took their roots from Simula – but not as an attempt to improve it. It was the promise of an entirely new way to structure computations that took my fancy. As it turned out, it would take quite a few years to understand how to use the insights and to devise efficient mechanisms to execute them.
Algol 60 allowed textually nested procedures and passing procedures as parameters (but not returning procedures as results). The requirement in the copying rule for systematic change of identifiers has the effect of enforcing static (that is lexicographic) binding of free variables.
In their book “Algol 60 Implementation”, Randell and Russell (1964, Sect. 2.2) handle this by two sets of links between stack frames. The dynamic chain links each stack frame, representing a procedure call, to the frame that called it. The static chain links each stack frame to that of the textually containing procedure, which might be much further away. Free variables are accessed via the static chain.
This mechanism works well for Algol 60 but in a language in which functions can be returned as results, a free variable might be held onto after the function call in which it was created has returned, and will no longer be present on the stack.
Landin (1964) solved this in his SECD machine. A function is represented by a closure, consisting of code for the function plus the environment for its free variables. The environment is a linked list of name-value pairs. Closures live in the heap.
Algol 60 is not normally thought of as a functional language but its rules for procedures (the Algol equivalent of functions) and variable binding were closely related to those of λ-calculus.
The Revised Report on Algol 60 (Naur 1963) is a model of precise technical writing. It defines the effect of a procedure call by a copying rule with a requirement for systematic change of identifiers where needed to avoid variable capture — exactly like β-reduction.
Although formal parameters could be declared value the default parameter passing mode was call by name, which required the actual parameter to be copied unevaluated into the procedure body at every occurrence of the formal parameter. This amounts to normal order reduction (but not graph reduction, there is no sharing). The use of call by name allowed an ingenious programming technique: Jensen’s Device.
Programming languages that existed when the concept of data abstraction arose did not support abstract data types, but some languages contained constructs that were precursors of this notion. …… The mechanism that matched the best was the class mechanism of Simula 67. A Simula class groups a set of procedures with some variables. A class can be instantiated to provide an object containing its own copies of the variables; the class contains code that initializes these variables at instantiation time. However, Simula classes did not enforce encapsulation ……, and Simula was lacking several other features needed to support data abstraction, …….
In this Standard the name SIMULA is considered synonymous with SIMULA 67. …… It is recommended that the language defined in this Standard be referred to as "Standard SIMULA".
SIMULA includes most of the ALGOL 60 language. Wherever ALGOL is used in this Standard it relates to the STANDARD ALGOL 60 definition (ISO 1538).
SIMSCRIPT was the only simulation language that we were closely acquainted with during the design phase of SIMULA. From the preceding sections it will be evident that it had a considerable impact through its list processing and time scheduling mechanisms. It also contained a set of random drawing and other utility routines, which served as a model for our procedure library.
the ALGOL-like call by name parameters were out of the question for reasons of security and storage allocation strategy: the actual parameter could be lost during the lifetime of an object. The problem then was to find a name-parameter-like mechanism that would guarantee a safe place for the actual parameter. After much trial and error we hit on the virtual quantity concept where the actual would have to be declared in the object itself, but at a deeper subclass level than that of the virtual specification. Now generalized objects could be defined whose behaviour pattern could be left partly unspecified in a prefix class body. Different subclasses could contain different actual procedure declarations.
In ALGOL, blocks (including procedures) are seen externally as generalized operations. By introducing mechanisms for quasi-parallel sequencing, essentially the same construct could play the role of processes in parallel, and through mechanisms for naming block instances and accessing their contents they could function as generalized data objects. The essential benefits of combining data and operations in a single construct were already there to be explored.
An object would start its life like an instance of a function procedure, invoked by the evaluation of a generating expression. During this phase the object might initialize its own local variables. Then, on passage through the end of the object or as the result of a new basic operation "detach", control would return to the generating expression delivering a reference to the object as the function value. In the former case the object was "terminated" with no further own actions, in the latter case it had become a "detached object" capable of functioning as a "coroutine".
The basic coroutine call "resume (<object reference>)" would make control leave the active object, leaving behind a reactivation point at the end of the resume statement, and enter the referenced object at its reactivation point.
C++ was designed to provide Simula’s facilities for program organization together with C’s efficiency and flexibility for systems programming.
Java is a blue collar language. It’s not PhD thesis material but a language for a job. Java feels very familiar to many different programmers because I had a very strong tendency to prefer things that had been used a lot over things that just sounded like a good idea. …… It has an object-oriented flavor that derives from a number of languages—Simula, C/C++, Objective C, Cedar/Mesa, Modula, and Smalltalk.
C++ was designed to provide Simula’s facilities for program organization together with C’s efficiency and flexibility for systems programming.
Java is a blue collar language. It’s not PhD thesis material but a language for a job. Java feels very familiar to many different programmers because I had a very strong tendency to prefer things that had been used a lot over things that just sounded like a good idea. …… It has an object-oriented flavor that derives from a number of languages—Simula, C/C++, Objective C, Cedar/Mesa, Modula, and Smalltalk.
On the other hand, if a procedurePis specified as virtual in a classCthe binding scheme is semi-dynamic. Any call forPoccurring inCor in any subclass ofCwill bind to that declaration ofPwhich occurs at the innermost prefix level of the actual object containing such a declaration (and similarly for remote accesses). Thus, the body of the procedurePmay, at the prefix level ofC, be postponed to occur in any subclass ofC. It may even be replaced by more appropriate ones in further subclasses.
This binding scheme is dynamic in the sense that it depends on the class membership of the actual object. But there is nevertheless a degree of compiler control; the access can be implemented as indirect through a table produced by the compiler forCand for each of its subclasses. ……
a virtual procedure can be seen as a parameter, where the actual parameter is a procedure residing safely within the object itself, at an appropriate prefix level. There is the additional advantage that the procedure has direct access to attributes of the object containing it.
The main impact of Simula 67 has turned out to be the very wide acceptance of many of its basic concepts: objects, but usually without own actions, classes, inheritance, and virtuals, often the default or only way of binding “methods”, (as well as pointers and dynamic object generation).
There is universal use of the term “object orientation”, OO. Although no standard definition exists, some or all of the above ideas enter into the OO paradigm of system development. There is a large flora of OO languages around for programming and system specification. …… The importance of the OO paradigm today is such that one must assume something similar would have come about also without the Simula effort. The fact remains, however, that the OO principle was introduced in the mid 60’s through these languages.
Simula 67 had an immediate success as a simulation language, and was, for instance extensively used in the design of VLSI chips, e.g. at INTEL. As a general programming language, its impact was enhanced by lectures at NATO Summer Schools given by OJD, materialized as a chapter in a book on structured programming. The latter has influenced research on the use of abstract data types, e.g., the CLU language, as well as research on monitors and operating system design.
A major new impact area opened with the introduction of workstations and personal computers. Alan Kay and his team at Xerox PARC developed Smalltalk, an interactive language building upon Simula’s objects, classes and inheritance. It is oriented towards organising the cooperation between a user and her/his personal computer.
In general attribute identifiers may be redeclared in subclasses, as is the case of inner blocks. The identity of an attribute is determined by the prefix level of the accessing occurrence, or, if the access is remote, by the class qualifying the object reference in question. In this way any ambiguity of identifier binding is resolved textually, i.e at compile time; we call it static binding.
During the summer and autumn of 1963, …… Instead Dahl and Nygaard introduced the far more powerful process concept which came to constitute the basic, unifying feature of the SIMULA I language. In short, a process can be understood as a generalized ALGOL procedure with quasi-parallel properties. ……
they became more and more preoccupied with the opportunities embedded in Tony Hoare's record class construct, first presented in ALGOL bulletin no. 21, 1965. …… What they were really looking for was some kind of generalized process concept with record class properties. The answer to their problem suddenly appeared in December 1966, when the idea of prefixing was introduced. A process, later called an object, could now be regarded as consisting of two layers: A prefix layer containing references to its predecessor and successor along with a number of other properties, and a main layer containing the attributes of the object in question. In addition to this important new feature, they also introduced the class concept, which can roughly be described as a highly refined version of SIMULA I's activity concept. This powerful new concept made it possible to establish class and subclass hierarchies of concatenated objects.
The system class Simulation introduces the notion of time. It means that if nested, there will be other local(nested) times.
SIMSCRIPT was the only simulation language that we were closely acquainted with during the design phase of SIMULA. From the preceding sections it will be evident that it had a considerable impact through its list processing and time scheduling mechanisms. It also contained a set of random drawing and other utility routines, which served as a model for our procedure library.
I wound up in graduate school at the University of Utah in the Fall of 1966, ……. …… The documentation was incomprehensible. Supposedly, this was the Case-Western Reserve 1107 Algol – but it had been doctored to make a language called Simula; the documentation read like Norwegian transliterated into English, which in fact it was. There were uses of words like activity and process that didn’t seem to coincide with normal English usage. ……
The weirdest part was the storage allocator, which did not obey a stack discipline as was usual for Algol. …… What Simula was allocating were structures very much like the instances of Sketchpad. There wee descriptions that acted like masters and they could create instances, each of which was an independent entity. What Sketchpad called masters and instances, Simula called activities and processes. Moreover, Simula was a procedural language for controlling Sketchpad-like objects, thus having considerably more flexibility than constraints (though at some cost in elegance) ……. ……
For the first time I thought of the whole as the entire computer and wondered why anyone would want to divide it up into weaker things called data structures and procedures. Why not divide it up into little computers, as time sharing was starting to? But not in dozens. Why not thousands of them, each simulating a useful structure? ……
It is not too much of an exaggeration to say that most of my ideas from then on took their roots from Simula – but not as an attempt to improve it. It was the promise of an entirely new way to structure computations that took my fancy. As it turned out, it would take quite a few years to understand how to use the insights and to devise efficient mechanisms to execute them.
Programming languages that existed when the concept of data abstraction arose did not support abstract data types, but some languages contained constructs that were precursors of this notion. …… The mechanism that matched the best was the class mechanism of Simula 67. A Simula class groups a set of procedures with some variables. A class can be instantiated to provide an object containing its own copies of the variables; the class contains code that initializes these variables at instantiation time. However, Simula classes did not enforce encapsulation ……, and Simula was lacking several other features needed to support data abstraction, …….
In this Standard the name SIMULA is considered synonymous with SIMULA 67. …… It is recommended that the language defined in this Standard be referred to as "Standard SIMULA".
SIMULA includes most of the ALGOL 60 language. Wherever ALGOL is used in this Standard it relates to the STANDARD ALGOL 60 definition (ISO 1538).
On the other hand, if a procedurePis specified as virtual in a classCthe binding scheme is semi-dynamic. Any call forPoccurring inCor in any subclass ofCwill bind to that declaration ofPwhich occurs at the innermost prefix level of the actual object containing such a declaration (and similarly for remote accesses). Thus, the body of the procedurePmay, at the prefix level ofC, be postponed to occur in any subclass ofC. It may even be replaced by more appropriate ones in further subclasses.
This binding scheme is dynamic in the sense that it depends on the class membership of the actual object. But there is nevertheless a degree of compiler control; the access can be implemented as indirect through a table produced by the compiler forCand for each of its subclasses. ……
a virtual procedure can be seen as a parameter, where the actual parameter is a procedure residing safely within the object itself, at an appropriate prefix level. There is the additional advantage that the procedure has direct access to attributes of the object containing it.
The main impact of Simula 67 has turned out to be the very wide acceptance of many of its basic concepts: objects, but usually without own actions, classes, inheritance, and virtuals, often the default or only way of binding “methods”, (as well as pointers and dynamic object generation).
There is universal use of the term “object orientation”, OO. Although no standard definition exists, some or all of the above ideas enter into the OO paradigm of system development. There is a large flora of OO languages around for programming and system specification. …… The importance of the OO paradigm today is such that one must assume something similar would have come about also without the Simula effort. The fact remains, however, that the OO principle was introduced in the mid 60’s through these languages.
Simula 67 had an immediate success as a simulation language, and was, for instance extensively used in the design of VLSI chips, e.g. at INTEL. As a general programming language, its impact was enhanced by lectures at NATO Summer Schools given by OJD, materialized as a chapter in a book on structured programming. The latter has influenced research on the use of abstract data types, e.g., the CLU language, as well as research on monitors and operating system design.
A major new impact area opened with the introduction of workstations and personal computers. Alan Kay and his team at Xerox PARC developed Smalltalk, an interactive language building upon Simula’s objects, classes and inheritance. It is oriented towards organising the cooperation between a user and her/his personal computer.
During the summer and autumn of 1963, …… Instead Dahl and Nygaard introduced the far more powerful process concept which came to constitute the basic, unifying feature of the SIMULA I language. In short, a process can be understood as a generalized ALGOL procedure with quasi-parallel properties. ……
they became more and more preoccupied with the opportunities embedded in Tony Hoare's record class construct, first presented in ALGOL bulletin no. 21, 1965. …… What they were really looking for was some kind of generalized process concept with record class properties. The answer to their problem suddenly appeared in December 1966, when the idea of prefixing was introduced. A process, later called an object, could now be regarded as consisting of two layers: A prefix layer containing references to its predecessor and successor along with a number of other properties, and a main layer containing the attributes of the object in question. In addition to this important new feature, they also introduced the class concept, which can roughly be described as a highly refined version of SIMULA I's activity concept. This powerful new concept made it possible to establish class and subclass hierarchies of concatenated objects.
C++ was designed to provide Simula’s facilities for program organization together with C’s efficiency and flexibility for systems programming.
Java is a blue collar language. It’s not PhD thesis material but a language for a job. Java feels very familiar to many different programmers because I had a very strong tendency to prefer things that had been used a lot over things that just sounded like a good idea. …… It has an object-oriented flavor that derives from a number of languages—Simula, C/C++, Objective C, Cedar/Mesa, Modula, and Smalltalk.
Algol 60 allowed textually nested procedures and passing procedures as parameters (but not returning procedures as results). The requirement in the copying rule for systematic change of identifiers has the effect of enforcing static (that is lexicographic) binding of free variables.
In their book “Algol 60 Implementation”, Randell and Russell (1964, Sect. 2.2) handle this by two sets of links between stack frames. The dynamic chain links each stack frame, representing a procedure call, to the frame that called it. The static chain links each stack frame to that of the textually containing procedure, which might be much further away. Free variables are accessed via the static chain.
This mechanism works well for Algol 60 but in a language in which functions can be returned as results, a free variable might be held onto after the function call in which it was created has returned, and will no longer be present on the stack.
Landin (1964) solved this in his SECD machine. A function is represented by a closure, consisting of code for the function plus the environment for its free variables. The environment is a linked list of name-value pairs. Closures live in the heap.
A procedure deviates from a block in that ⑴ it has a name and ⑵ may be referred to at several different places in the program, and ⑶ that parameters may be given to it when invoked. A procedure shares the property of a block that it is impossible to establish a reference to it or to its interior.
Algol 60 is not normally thought of as a functional language but its rules for procedures (the Algol equivalent of functions) and variable binding were closely related to those of λ-calculus.
The Revised Report on Algol 60 (Naur 1963) is a model of precise technical writing. It defines the effect of a procedure call by a copying rule with a requirement for systematic change of identifiers where needed to avoid variable capture — exactly like β-reduction.
Although formal parameters could be declared value the default parameter passing mode was call by name, which required the actual parameter to be copied unevaluated into the procedure body at every occurrence of the formal parameter. This amounts to normal order reduction (but not graph reduction, there is no sharing). The use of call by name allowed an ingenious programming technique: Jensen’s Device.
the ALGOL-like call by name parameters were out of the question for reasons of security and storage allocation strategy: the actual parameter could be lost during the lifetime of an object. The problem then was to find a name-parameter-like mechanism that would guarantee a safe place for the actual parameter. After much trial and error we hit on the virtual quantity concept where the actual would have to be declared in the object itself, but at a deeper subclass level than that of the virtual specification. Now generalized objects could be defined whose behaviour pattern could be left partly unspecified in a prefix class body. Different subclasses could contain different actual procedure declarations.
In general attribute identifiers may be redeclared in subclasses, as is the case of inner blocks. The identity of an attribute is determined by the prefix level of the accessing occurrence, or, if the access is remote, by the class qualifying the object reference in question. In this way any ambiguity of identifier binding is resolved textually, i.e at compile time; we call it static binding.
In ALGOL, blocks (including procedures) are seen externally as generalized operations. By introducing mechanisms for quasi-parallel sequencing, essentially the same construct could play the role of processes in parallel, and through mechanisms for naming block instances and accessing their contents they could function as generalized data objects. The essential benefits of combining data and operations in a single construct were already there to be explored.
An object would start its life like an instance of a function procedure, invoked by the evaluation of a generating expression. During this phase the object might initialize its own local variables. Then, on passage through the end of the object or as the result of a new basic operation "detach", control would return to the generating expression delivering a reference to the object as the function value. In the former case the object was "terminated" with no further own actions, in the latter case it had become a "detached object" capable of functioning as a "coroutine".
The basic coroutine call "resume (<object reference>)" would make control leave the active object, leaving behind a reactivation point at the end of the resume statement, and enter the referenced object at its reactivation point.
The procedure "call" is not a part of the Common Base, but is a natural part of a SIMULA 67 Common Base implementation. ……This definition of call is tentative, since the problem is currently being studied by a Technical Committee under the SIMULA Standards Group.
The system class Simulation introduces the notion of time. It means that if nested, there will be other local(nested) times.
I wound up in graduate school at the University of Utah in the Fall of 1966, ……. …… The documentation was incomprehensible. Supposedly, this was the Case-Western Reserve 1107 Algol – but it had been doctored to make a language called Simula; the documentation read like Norwegian transliterated into English, which in fact it was. There were uses of words like activity and process that didn’t seem to coincide with normal English usage. ……
The weirdest part was the storage allocator, which did not obey a stack discipline as was usual for Algol. …… What Simula was allocating were structures very much like the instances of Sketchpad. There wee descriptions that acted like masters and they could create instances, each of which was an independent entity. What Sketchpad called masters and instances, Simula called activities and processes. Moreover, Simula was a procedural language for controlling Sketchpad-like objects, thus having considerably more flexibility than constraints (though at some cost in elegance) ……. ……
For the first time I thought of the whole as the entire computer and wondered why anyone would want to divide it up into weaker things called data structures and procedures. Why not divide it up into little computers, as time sharing was starting to? But not in dozens. Why not thousands of them, each simulating a useful structure? ……
It is not too much of an exaggeration to say that most of my ideas from then on took their roots from Simula – but not as an attempt to improve it. It was the promise of an entirely new way to structure computations that took my fancy. As it turned out, it would take quite a few years to understand how to use the insights and to devise efficient mechanisms to execute them.
On the other hand, if a procedurePis specified as virtual in a classCthe binding scheme is semi-dynamic. Any call forPoccurring inCor in any subclass ofCwill bind to that declaration ofPwhich occurs at the innermost prefix level of the actual object containing such a declaration (and similarly for remote accesses). Thus, the body of the procedurePmay, at the prefix level ofC, be postponed to occur in any subclass ofC. It may even be replaced by more appropriate ones in further subclasses.
This binding scheme is dynamic in the sense that it depends on the class membership of the actual object. But there is nevertheless a degree of compiler control; the access can be implemented as indirect through a table produced by the compiler forCand for each of its subclasses. ……
a virtual procedure can be seen as a parameter, where the actual parameter is a procedure residing safely within the object itself, at an appropriate prefix level. There is the additional advantage that the procedure has direct access to attributes of the object containing it.
The main impact of Simula 67 has turned out to be the very wide acceptance of many of its basic concepts: objects, but usually without own actions, classes, inheritance, and virtuals, often the default or only way of binding “methods”, (as well as pointers and dynamic object generation).
There is universal use of the term “object orientation”, OO. Although no standard definition exists, some or all of the above ideas enter into the OO paradigm of system development. There is a large flora of OO languages around for programming and system specification. …… The importance of the OO paradigm today is such that one must assume something similar would have come about also without the Simula effort. The fact remains, however, that the OO principle was introduced in the mid 60’s through these languages.
Simula 67 had an immediate success as a simulation language, and was, for instance extensively used in the design of VLSI chips, e.g. at INTEL. As a general programming language, its impact was enhanced by lectures at NATO Summer Schools given by OJD, materialized as a chapter in a book on structured programming. The latter has influenced research on the use of abstract data types, e.g., the CLU language, as well as research on monitors and operating system design.
A major new impact area opened with the introduction of workstations and personal computers. Alan Kay and his team at Xerox PARC developed Smalltalk, an interactive language building upon Simula’s objects, classes and inheritance. It is oriented towards organising the cooperation between a user and her/his personal computer.
useful properties from the standpoint of concept modelling. ……
⑵ Decomposition. A block where only local quantities are referenced is a completely selfcontained program component, which will function as specified in any context. Through a procedure heading a block (procedure) instance may interact with a calling sequence. Procedures which reference or change non-local variables represent a partial decomposition of the total task, which is useful for direct interaction with the program environment.
Algol 60 allowed textually nested procedures and passing procedures as parameters (but not returning procedures as results). The requirement in the copying rule for systematic change of identifiers has the effect of enforcing static (that is lexicographic) binding of free variables.
In their book “Algol 60 Implementation”, Randell and Russell (1964, Sect. 2.2) handle this by two sets of links between stack frames. The dynamic chain links each stack frame, representing a procedure call, to the frame that called it. The static chain links each stack frame to that of the textually containing procedure, which might be much further away. Free variables are accessed via the static chain.
This mechanism works well for Algol 60 but in a language in which functions can be returned as results, a free variable might be held onto after the function call in which it was created has returned, and will no longer be present on the stack.
Landin (1964) solved this in his SECD machine. A function is represented by a closure, consisting of code for the function plus the environment for its free variables. The environment is a linked list of name-value pairs. Closures live in the heap.
Algol 60 is not normally thought of as a functional language but its rules for procedures (the Algol equivalent of functions) and variable binding were closely related to those of λ-calculus.
The Revised Report on Algol 60 (Naur 1963) is a model of precise technical writing. It defines the effect of a procedure call by a copying rule with a requirement for systematic change of identifiers where needed to avoid variable capture — exactly like β-reduction.
Although formal parameters could be declared value the default parameter passing mode was call by name, which required the actual parameter to be copied unevaluated into the procedure body at every occurrence of the formal parameter. This amounts to normal order reduction (but not graph reduction, there is no sharing). The use of call by name allowed an ingenious programming technique: Jensen’s Device.
