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Values and Objects in Programming Languages.

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Abstract

The design of the enumeration data type is known to be imperfect in both PASCAL and ADA. A new design which avoids the known problems is proposed. An implementation in the experimental programming language CONCISE confirms the design.
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VALUES AND OBJECTS IN PROGRAMMING LANGUAGES*
B. J. MacLennan
Computer Science Department
NavalPostgraduate School
Monterey, CA93943
1. INTRODUCTION
The terms value-oriented and object-oriented are used to describe both programming languages and
programming styles. This paper attempts to elucidate the differences between values and objects and
argues that their proper discrimination can be a valuable aid to conquering program complexity.The first
section shows that values amount to timeless abstractions for which the concepts of updating, sharing and
instantiation have nomeaning. The second section shows that objects exist in time and, hence, can be
created, destroyed, copied, shared and updated. The third section argues that proper discrimination of these
concepts in programming languages will clarify problems such as the role of state in functional
programming. The paper concludes by discussing the use of the value/object distinction as a tool for
program organization.
2. VALUES
Values areapplicative. The term value-oriented is most often used in conjunction with applicative
programming. That is, with programming with pure expressions and without the use of assignment or other
imperative facilities. Another waytoput this is that value-oriented programming is programming in the
absence of side-effects. This style of programming is important because it has manyofthe advantages of
simple algebraic expressions, viz. that an expression can be understood by understanding its constituents,
that the meaning of the subexpressions is independent of their context, and that there are simple interfaces
between the parts of the expression that are obvious from the syntax of the expression. That is, each part of
an expression involving values is independent of all the others. One reason for this is that values are read-
only, i.e., it is not possible to update their components. Since theyare unchangeable, it is always safe to
share values for efficiency. That is, theyexhibit referential transparency: there is neverany danger of one
expression altering something which is used by another expression. Anysharing that takes place is hidden
from the programmer and is done by the system for more efficient storage utilization. Av oiding updating
eliminates dangling reference problems and simplifies deallocation [3].
What arevalues? We hav e discussed a number of properties of values. What exactly are they? The best
examples of values are mathematical entities, such as integer,real and complexnumbers, hence we can
understand values better by understanding these better.
One characteristic of mathematical entities is that theyare atemporal, in the literal sense of being timeless.
To put it another way,the concept of time or duration does not apply to mathematical entities anymore than
the concept color applies to them; theyare neither created nor destroyed. When we write 2 +3there is no
implication that 5 has just come into existence and that 2 and 3 have been consumed. What is it about
numbers that give them this property?
Values areabstractions. The fundamental fact that givesmathematical entities and other values their
properties is that theyare abstractions, (universals or concepts). Although a full explication of
mathematical entities is beyond the scope of this paper it should be fairly clear that the number 2 is an
abstraction that subsumes all particular pairs. This universal nature of abstractions makes them atemporal,
or timeless. The number 2 can neither be created nor destroyed because its existence is not tied to the
creation or destruction of particular pairs. Indeed, the concept of existence, in its usual sense, is not
applicable to the number 2. It is the same with all values, because all values correspond to abstractions;
*The work reported herein was supported by the Foundation Research Program of the NavalPostgraduate School with funds
provided by the Chief of NavalResearch. This paper is a condensation and revision of [6], to which the reader is referred for
further information.
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theycan neither be created nor destroyed.
It is also the case that abstractions, and hence values, are immutable. Although values can be operated on,
in the sense of relating values to other values, theycannot be altered. That is, 2 +1=3states a relation
among values; it does not alter them. When in a programming language we assign xthe value 2, x:=2,
and later add one to x,x:=x+1, haven’twechanged a number,which is a value? No, we haven’t; the
number 2 has remained the same. What we have changed is the number that the name ‘x’denotes. Wecan
give names to values and we can change the names that we give tovalues, but this doesn’tchange the
values. The naming of values and the changing of names is discussed in a later section.
Values cannot be counted. Acorollary of the above isthat there is not such a thing as ‘‘copies’’ofavalue.
This should be clear from mathematics: it is not meaningful to speak of this 2 or that 2; there is just 2. That
is, the number 2 is uniquely determined by its value. This is because an abstraction is uniquely determined
by the things which it subsumes, hence, anything which subsumes all possible pairs is the abstraction 2.
Therefore, the concept ‘number’ is not evenapplicable to abstractions; it makes no sense to ask howmany
2’sthere are. In a programming language, it is pointless to makeanother copyofavalue; there is no such
thing. There is also no reason to makesuch a copysince values are immutable. (It is, of course, possible to
makecopies of a representation of a value; this is discussed later.)
It is also meaningless to talk about the sharing of values. Since values are immutable, cannot be counted,
and cannot be copied, it is irrelevant whether different program segments share the same value or different
‘‘copies’’ofthe value. Of course, there might be implementation differences. If along string value is
assigned to a variable it will makeabig difference whether a fresh copymust be made or whether a pointer
to the original copycan be stored. While this is an important implementation concern, it is irrelevant to the
semantics of values.
Values areused to model abstractions. We hav e discussed a number of the characteristics of values but
have not discussed whether values should be included in programming languages, or,iftheyare, what they
should be used for.The answer to this question lies in the relation we have shown between values and
abstractions: values are the programming language equivalent of abstractions. Thus, values will be most
effective when theyare used to model abstractions in the problem to be solved. This is in fact their usual
use, since integer and real data values are used to model quantities represented by integer and real
numbers. Similarly,the abstraction ‘color’ might be modeled by values of a Pascal or Ada enumeration
type, (RED, BLUE, GREEN). On the other hand, it is not common to treat compound data values, such as
complexnumbers or sequences, as values. If done, this would eliminate one source of errors, namely,
updating a data structure that is unknowingly shared [3]. Value-oriented languages, such as the languages
for data-flowmachines and functional programming, have only values. Is there anyneed for objects at all?
This is answered in the following sections.
3. OBJECTS
Computing can be viewed as simulation. It has been said that computing can be viewed as simulation [4].
This is certainly obvious in the case of programs that explicitly simulate or model some physical situation.
The metaphor can be extended to manyother situations. Consider an employee data base: each record in
the data base corresponds to an employee. The data base can be said to be a simulation, or model, of some
aspects of the corporation. Similarly,the data structures in an operating system often reflect the status of
some objects in the real world. For instance, theymight reflect the fact that a tape drive isrewinding or in a
parity-error status. The data structures can also reflect logical situations, such as the fact that a tape drive is
assigned to a particular job in the system.
Adata structureisneeded for eachentity. Simulation is simplified if there is a data structure corresponding
to each entity to be simulated, since this factors and encapsulates related information. This is exactly the
approach that has been taken in object-oriented programming languages, such as Smalltalk [4,7]. The usual
waytostructure a program in such a language is to create an object for each entity in the system being
modeled. These entities might be real-world objects or objects that are only real to the application, such as
figures on a display screen. The messages these objects respond to are just the relevant manipulations that
can be performed on the corresponding real entities. Giventhis relationship between programming
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language objects and real world objects, we will try to clarify the notion of an object.
What is an object? In our programming environment we have objects and in the real world we have
objects. Just what is an object? When we attempt to answer this question we immediately find ourselves
immersed in age-old philosophical problems. In particular,what makes one object different from another?
One philosophical answer to this question is to say that while the twoobjects have the same form, theyhav e
different substance. To put this is more concrete terms, we could say that the twoobjects are alikeinevery
wayexcept that theyoccupydifferent regions of space.
We find exactly the same situation arising in programming languages. We might have two array variables
that contain exactly the same values, yet theyare twodifferent variables. What makes them different?
Theyoccupydifferent locations. So by analogy,the form of the array variable is the order and value of its
elements while its substance is the region of memory it occupies.
Objects can be instantiated. There is also a less philosophical way in which we distinguish real world
objects: we give them proper names. We find an exactly analogous situation in programming languages.
Programming objects, such as the array variables already mentioned, generally have a unique name: the
reference to the object. This is generally closely related to the region of storage the object occupies. This
is not necessary,howev er, aswecan see by considering a file system. It is easy to see that files are objects;
it is quite normal to have two different files with the same contents. Of course, if the files are to be
distinguished, then theymust have distinct names. Forour purposes, we will not be too concerned about
what individuating element is used to distinguish objects; whether it is some form of unique identification
(such as a capability), or whether it is implicit in the region of storage occupied; we will assume that each
object is different from every other object eveniftheycontain the same data values. In general, we can say
that the uniqueness of an object is determined by its external relations and is independent of its internal
relations and properties. This is opposed to a value, which is completely determined by its internal
relations and properties (e.g., a set is completely determined by its elements). Thus there might be any
number of instances of otherwise identical objects. This leads to a number of further consequences.
Objects can be changed. We hav e said that the identity of an object is independent of anyofits internal
properties or attributes. For instance, evenifall of the elements of an array variable are changed, it is still
the same variable (because it occupies the same region of storage). This is of course likereal world objects,
for theytoo can change and retain their identity.Values, on the other hand can neverchange. For instance,
if we add 5 to 1 +2i,wedon’tchange 1 +2i,wecompute a different value, 6 +2i.This changeability,this
fact that an object might have one set of properties at one time and a different set at another time, is a
distinctive feature of objects (and of programs).
Objects have state. This changeability of objects leads to the idea of the state of an object: the sum total of
the internal properties and attributes of an object at a givenpoint in time. Thus, we can say that the state of
an object can be changed in time. State is of course a central idea in computer science, so it is not
surprising to find that objects are at the heart of computer science. Since the state of an object can change
in time, it is certainly the case that objects exist ‘‘in time,’’ i.e., theyare not atemporal likevalues.
Objects can be created and destroyed. The fact that objects are not atemporal leads to the conclusion that
theycan be both created and destroyed. This is familiar in programming languages where, for instance, a
variable might be created every time a certain block is entered and destroyed every time it is exited. Many
languages also provide explicit means for creating and destroying objects (e.g., Pascal’s‘new’ and
‘dispose’). Since values are atemporal, it is meaningless to speak of their being either created or destroyed.
Objects can be shared. Since there can be anynumber of instances of otherwise identical objects and since
objects can change their properties in time, it is a crucial question whether an object is shared or not. This
is because a change made to the object by one sharer will be visible to the other sharer.Such side-effects
are common in programming and are often used by programmers as a way of communicating. People also
frequently use shared objects as means of communication. Forinstance, twopersons might communicate
by altering the state of a blackboard.
Recall that in our discussion of values we found that the issue of sharing didn’tapply.Whether a particular
implementation chooses to share copies of values or not is irrelevant to the semantics of the program; it is
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strictly an issue of efficiency. Sharing is a crucial issue where objects are concerned.
Computer science as objectified mathematics. We can see nowanimportant difference between the domain
of mathematics and the domain of computer science. Mathematics deals with things such as numbers,
functions, vectors, groups, etc. These are all abstractions, i.e., values. It has been said that the theorems of
mathematics are timeless, and this is literally true. Since mathematics deals with the relations among
values and since values are atemporal, the resulting relations (which are themselves abstractions and
values) are atemporal. Conversely,much of computer science deals with objects and with the way they
change in time. State is a central idea. It might not be unreasonable to call computer science objectified
mathematics, or object-oriented mathematics.
It has frequently been observed that the advantage of applicative programming is that it is more
mathematical and eliminates the idea of state from programming. We can see that this means that
applicative programming deals only with values (indeed, several languages for applicative programming are
called ‘‘value-oriented’’languages). Really,applicative programming is just mathematics.
These ideas can be summarized in twoobservations:
Programming is object-oriented mathematics.
Mathematics is value-oriented programming.
These twoprinciples showthe unity between the twofields and isolate their differences.
4. VALUES AND OBJECTS IN PROGRAMMING LANGUAGES
Most languagesconfuse them. Both values and objects are accommodated in most programming
languages, although usually in a very asymmetric and ad hoc way. For example, a language such as
FORTRAN supports values of several types, including integers, reals, complexnumbers and logical values.
These are all treated as mathematical values; for example, it is not possible to ‘‘update’’the real part of a
complexnumber separately from the imaginary part. Of course, it is possible to store all of these values in
variables, but that is a different issue, as we will see later.Onthe other hand, FORTRAN provides objects
in the form of updatable, sharable arrays. This pattern has been followed with fewvariations in most other
languages. All of these languages unnecessarily tie the value or object nature of a thing to its type, usually
by treating the atomic data types as values and the compound data types as objects. We will argue below,
that this confusion complicates programming.
Programming languages are most often deficient in their treatment of compound values; in particular,they
rarely provide recursive data types as Hoare described them [3]. Theytend to confuse the logical issue of
whether a thing should be an object (i.e., it is shared, updatable, destroyable, etc.) with the implementation
issue of whether it should be shared for efficiency. Wewill see howthis can be solved later.
Mathematics deals poorly with objects. We hav e said that mathematics is value-oriented; that is, it deals
with timeless relations and operations on abstractions. Concepts that are central to objects (and computer
science), such as state, updating and sharing, are alien to mathematics. This is not to say that it is
impossible to deal with objects in mathematics; it is done every day,only indirectly.For instance, it is
common to deal in physics with systems that change in time; theyare represented mathematically by
functions of an independent variable representing time. The relationships between objects can be
represented as differential equations (or difference equations if state changes are quantized). Similarly,
mathematics can distinguish instances of an object by attaching a unique name (generally a natural number)
to each instance of a value. These techniques work but are awkward. A more fully developed attempt to
apply the concepts of mathematics to the description of objects can be seen in denotational semantics. Here
the state is explicitly passed from function to function to represent its alteration in time.
Fentheory deals poorly with values. In our fen theory [5] we attempted to deal with objects more directly
by developing an axiomatic theory of objects. This was done in twoways: (1) we discarded the axiom of
set theory that forces twosets with the same values to be identical. This permitted multiple instances of the
same set. (2) An axiom was inserted that required there to be at least a countable infinity of instances of
each set. The result was an object-oriented theory of sets and relations. This worked well for describing
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manyofthe properties of objects and for defining the semantics of those programming language constructs
that are object-oriented. Unfortunately,itsuffered from the dual problem of mathematics: it was awkward
to deal with values. What is 2? Is it the name of some distinguished object that we have chosen to
represent 2 or does it denote anyobject with a certain structure? There are related problems with
operations on values. For instance, which 5 does 2 +3return? These are all problems of attempting to deal
with values in an object-oriented system. Values are inherently extensional while fen theory is inherently
intensional (see [2], p. 109). The solution adopted in fen theory was to treat values as equivalence classes
of objects in the supporting logic. This was possible because that logic was extensional (i.e., value-
oriented).
Computersuse objects to represent values. These are exactly the issues that must be faced in dealing with
values on a computer.Abstractions are not physical objects (except so far as theyexist in our brains), so to
deal with them theymust be represented or encoded into objects. We dothis when we represent the
number 2 by the numeral ‘2’ or the word ‘two’ on a piece of paper.Once a value has been represented as
an object it acquires some of the attributes of objects. Clearly,wheneveravalue is to be manipulated in a
computer it must be represented as the state of some physical object. Typically,there will be manysuch
representing objects in a computer at a time. Forinstance, 2 can be represented by a bit pattern in a register
and in several memory locations. Therefore, everything ‘‘in’’acomputer is an object; there are no values in
computers. This does not imply,howev er, that values should be discarded from programming languages.
Programmersneed values. Most conventional languages have both values and objects, although a purely
object-oriented programming language could be designed. This could be done by storing everything in
memory and then only dealing with the addresses of these things. It would be likehaving a pointer to every
object. It would then be necessary for the programmer to keep track of the different instances of what were
intuitively the same value so that he wouldn’taccidently update a shared value or miss considering as equal
twoinstances of the same value. Some languages actually come close to this, such as Smalltalk. Unless
such a language were carefully designed, it would be almost impossible to deal with values such as
numbers in the usual way.
Programmersneed objects. Conversely,programmers need objects in their programming languages. There
have,for sure, been completely value-oriented programming languages. These include the FP and FFP
systems of Backus [1]. It is interesting to note, however, that Backus went on to define the AST system,
which includes the notion of state (and implicitly,ofobjects). Applicative languages were originally
developed in reaction to what was surely an overuse of objects and imperative features in programming.
Yet, it seems clear that we cannot eliminate them from programming without detrimental effect. For
example, it is not uncommon to see applicative programs pass large data structures, which represent the
state of the computation, from one function to the next. The result in such a case is not greater clarity,but
less. Weshould not be surprised to have todeal with objects in programming; as we argued before, this is a
natural outgrowth of the fact that we are frequently modeling real world objects. Abetter solution than
banning objects is to determine their proper application and discipline their use.
We should use appropriate modeling tools. This suggests that programmers should be clear about what
theyare trying to model and then use the appropriate constructs. If theyare modeling an abstraction, such
as a number,then theyshould use values; if theyare modeling an entity or thing that exists in time, then
theyshould use an object. This implies that languages should support both values and objects and the
means to use them in these ways. Toput it another way,wemust develop an appropriate discipline for
using values and objects and linguistic means for supporting that discipline.
Names should be fixed. Howcan we arrive atsuch a discipline? Howcan we tame the state? One of the
motivations for value-oriented programming is the incredible complexity that can result from a state
composed of hundreds or thousands of individual variables, all capable of being changed (the Von
Neumann bottleneck). We can see a possible solution to this problem by looking at natural languages.
Generally,aword has a fixed meaning within a givencontext. This holds whether the word is a common
noun or a proper name. We donot use a word to refer to one abstraction one moment and another the next,
or to refer to one object one moment and another the next. Yet this is exactly what we do with variables in
programming languages. To the extent that we need temporary identifiers, natural languages provide
pronouns. These are automatically bound and have a very limited scope (generally a sentence or two).
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Can these ideas be applied to programming languages? It would seem so; let’sconsider the consequences.
Suppose that names in programming languages were always bound to a fixed value or object within a
context; effectively all names would be constants. Similarly,wheneveranobject was created it could be
givenaname that would refer to that object until it was destroyed. There would be no ‘‘variables’’that can
be rebound from moment to moment by an assignment statement. Variables in the usual sense would only
be allowed as components of the state of an object and the only allowable assignments would be to these
components.
Would it be possible to program in such a language, or would it be too inconvenient? Without actually
designing it is difficult to tell. We can only point to the fact that a considerable amount of good
mathematics has been done without the aid of variables, not to mention a considerable amount dealing with
real world objects. Such a language could provide, as does mathematics, mechanisms for declaring
constants of very local scope. Some languages do provide these mechanisms already (e.g., ‘let
t=(a+b)/2 in ...’, or ‘sin(t)+cos(t)where t=...’). As suggested by natural languages, it might be
possible to provide some sort of pronoun facility.Hence, what we are describing is a programming
language that is variable-free, but does not do awaywith objects, values, or names.
5. CONCLUSIONS
In this paper we have distinguished the twoconcepts ‘value’ and ‘object’. We hav e shown that values are
abstractions, and hence atemporal, unchangeable and non-instantiated. We hav e shown that objects
correspond to real world entities, and hence exist in time, are changeable, have state, are instantiated, and
can be created, destroyed, and shared. These concepts are implicit in most programming languages, but are
not well delimited.
We claim that programs can be made more manageable by recognizing explicitly the value/object
distinction. This can be done by incorporating facilities for handling values and objects in programming
languages.
REFERENCES
[1] Backus, J., Can programming be liberated from the Von Neumann style? Afunctional style and its
algebra of programs, CACM 21, 8, August 1978, pp. 613-641.
[2] Flew, A., ADictionary of Philosophy, St. Martin’sPress, NewYork, 1979.
[3] Hoare, C.A.R., Recursive Data Structures, Stanford University Computer Science Department
Technical Report STAN-CS-73-400 and Stanford University A.I. Lab.MEMO AIM-223, October
1973.
[4] Kay,Alan C., Microelectronics and the personal computer, Scientific American 237, 3, September
1977, pp. 230-244.
[5] MacLennan, B. J., Fen - an axiomatic basis for program semantics, CACM 16, 8, August 1973, pp.
468-471.
[6] MacLennan, B. J., Values and Objects in Programming Languages, NavalPostgraduate School
Computer Science Department Technical Report NPS52-81-006, April 1981.
[7] Schoch, J., An overviewofthe language Smalltalk-72, Sigplan Notices 14, 9, September 1979, pp.
64-73.
... Objects, values, and types. I agree on most points with the classic paper [48]: both values and objects and a clear distinction between them are useful in programming. I would like to have both objects (concrete) and values (abstract) of all types, i.e., the abstract-concrete dichotomy should be independent of type. ...
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The power and convenience of a programming language may be enhanced for certain applications by permitting treelike data structures to be defined by recursion. This paper suggests a pleasing notation by which such structures can be declared and processed; it gives the axioms which specify their properties, and suggests an efficient implementation method. It shows how a recursive data structure may be used to represent another data type, for example, a set. It then discusses two ways in which significant gains in efficiency can be made by selective updating of structures, and gives the relevant proof rules and hints for implementation. The examples show that a certain range of applications in symbol manipulation can be efficiently programmed without introducing the low-level concept of a reference into a high-level programming language.
Chapter
Conventional programming languages are growing ever more enormous, but not stronger. Inherent defects at the most basic level cause them to be both fat and weak: their primitive word-at-a-time style of programming inherited from their common ancestor—the von Neumann computer, their close coupling of semantics to state transitions, their division of programming into a world of expressions and a world of statements, their inability to effectively use powerful combining forms for building new programs from existing ones, and their lack of useful mathematical properties for reasoning about programs. An alternative functional style of programming is founded on the use of combining forms for creating programs. Functional programs deal with structured data, are often nonrepetitive and nonrecursive, are hierarchically constructed, do not name their arguments, and do not require the complex machinery of procedure declarations to become generally applicable. Combining forms can use high level programs to build still higher level ones in a style not possible in conventional languages. Associated with the functional style of programming is an algebra of programs whose variables range over programs and whose operations are combining forms. This algebra can be used to transform programs and to solve equations whose “unknowns” are programs in much the same way one transforms equations in high school algebra. These transformations are given by algebraic laws and are carried out in the same language in which programs are written. Combining forms are chosen not only for their programming power but also for the power of their associated algebraic laws. General theorems of the algebra give the detailed behavior and termination conditions for large classes of programs. A new class of computing systems uses the functional programming style both in its programming language and in its state transition rules. Unlike von Neumann languages, these systems have semantics loosely coupled to states—only one state transition occurs per major computation.
An overview of the language Smalltalk-72
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