crowl@cs.rochester.edu (Lawrence Crowl) (02/27/88)
Oberon is a new language designed by Niklaus Wirth. It is a refinement of
Modula-2. The remainder of this article is the LaTeX form of an article
which appeared in ASCII form on BIX. Enjoy.
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\begin{document}
\title{From Modula to Oberon}
\author{N. Wirth}
\date{Tuesday 23 February 1988}
\maketitle
\begin{abstract}
The programming language Oberon is the result of a
concentrated effort increase the power of Modula-2 and
simultaneously to reduce its complexity. Several features
were eliminated, and a few were added in order to increase
the expressive power and flexibility of the language. This
paper describes and motivates the changes. The language is
defined in a concise report.
\vspace{0.25in}
\noindent
BIX oberon/long.messages \#11,
from frode, 21774 chars, Tue Feb 23 20:43:46 1988.
\break
TITLE: Oberon Language Article, (c) ETH-ZENTRUM, SWITZERLAND
\end{abstract}
\section*{Introduction}
The programming language Oberon evolved from a project whose
goal was the design of a modern, flexible and efficient
operating system for a single-user workstation. A
principal guideline was to concentrate on properties that
are genuinely essential and --- as a consequence --- to omit
ephemeral issues. It is the best way to keep a system in
hand, to make it understandable, explicable, reliable and
efficiently implementable.
Initially, it was planned to express the system in Modula-2
[1], as that language supports the notion of modular design
quite effectively, and because an operating system has to
be designed in terms of separately compilable parts with
conscientiously chosen interfaces. In fact, an operating
system should be no more than a set of basic modules, and
the design of an application must be considered as a
goal-oriented extension of that basic set: Programming is
always {\em extending} a given system.
Whereas modern languages, such as Modula, support the notion
of extensibility in the procedural realm, the notion is
less well established in the domain of data types. Modula
in particular does not allow the definition of new data
types as extensions of other, programmer-defined types in an
adequate manner. An additional feature was called for,
thereby giving rise to an extension of Modula.
The concept of the planned operating system also called for
a highly dynamic, centralized storage management relying on
the technique of garbage collection. Although Modula does
not prevent the incorporation of a garbage collector in
principle, its variant record feature constitutes a genuine
obstacle. As the new facility for extending types would
make the variant record feature superfluous, the removal of
this stumbling block was a logical decision. This step,
however gave rise to a restriction (subset) of Modula.
It soon became clear that the rule to concentrate on the
essential and to eliminate inessential should not only be
applied to the design of the new system, but equally
stringently to the language in which the system is
formulated. The application of the principle thus led from
Modula to a new language. The adjective new, however, has
to be understood in proper context: Oberon evolved from
Modula by very few additions and several subtractions. In
relying on evolution rather than revolution we remain in the
tradition of a long development that led from Algol to
Pascal, then to Modula-2, and eventually to Oberon. The
common trait of these languages are their procedural rather
than functional model, and the strict typing of data. More
fundamental even is perhaps the idea of abstraction: the
language must be defined in terms of mathematical, abstract
concepts without reference to any computing mechanism. Only
if a language satisfies this criterion can it be called
``higher-level''. No syntactic coasting whatsoever can earn a
language this attribute alone.
The definition of a language must be coherent and concise.
This can only be achieved by a careful choice of the
underlying abstractions an appropriate structure combining
them. The language manual must be reasonably short,
avoiding the explanation of individual cases derivable from
the general rules. The power of a formalism must not be
measured by the length of its description. To the contrary,
an overly lengthy definition is a sure symptom of
inadequacy. In this respect, not complexity but simplicity
must be the goal.
In spite of its brevity, a description must be complete.
Completeness is to be achieved within the framework of the
chosen abstractions. Limitations imposed by particular
implementations do not belong to a language definition
proper. Examples of such restrictions are the maximum
values of numbers, rounding and truncation errors in
arithmetic, and actions taken when a program violates the
stated rules. It should not be necessary to supplement a
language definition with voluminous standards document to
cover ``unforeseen'' cases.
But neither should a programming language be a mathematical
theory only. It must be practical tool. This imposes
certain limits on the terseness of the formalism. Several
features of Oberon are superfluous from a purely theoretical
point of view. They are nevertheless retained for practical
reasons, either for programmers' convenience or to allow
for efficient code generation without the necessity of
complex, ``optimizing'' pattern matching algorithms in
compilers. Examples of such features are the presence of
several forms of repetitive statements, and of standard
procedures such as INC, DEC, and ODD. They complicate
neither the language conceptually nor the compiler to any
significant degree.
These underlying premises must be kept in mind when
comparing Oberon with other languages.
Neither the language
nor its defining document reach the ideal; but Oberon
approximates these goals much better than its predecessors.
A compiler for Oberon has been implemented for the NS32000
processor family and is embedded in the Oberon operating
environment. The following data provide an estimate of the
simplicity and efficiency of the implementation, and readers
are encouraged to compare them with implementations of other
languages. (Measurements with 10 MHz NS32032).
\begin{center}
\begin{tabular}{lrrrr}
\hline
&\multicolumn{2}{c}{length of}
&\multicolumn{1}{c}{length of}
&\multicolumn{1}{c}{time of self}\\
&\multicolumn{2}{c}{source program}
&\multicolumn{1}{c}{compiled code}
&\multicolumn{1}{c}{compilation}\\
\hline
&lines &characters &bytes &seconds\\
\hline
Parser&1116&3671&99928&11.53\\
Scanner&346&9863&3388&3.80\\
Import/Export&514&18386&4668&5.25\\
Code generator&19636&5901&21636&21.02\\
Total&3939&130869&39620&41.60\\
\end{tabular}
\end{center}
Subsequently, we present a brief introduction to Oberon
assuming familiarity with Modula (or Pascal), concentrating
on the added features and listing the eliminated ones. In
order to be able to ``start with a clean table'', the latter
are taken first.
\section*{Features omitted from Modula}
\subsection*{Data types}
Variant records are eliminated, because they constitute a
genuine difficulty for the implementation of a reliable
storage management system based on automatic garbage
collection. The functionality of variant records is
preserved by the introduction of extensible data types.
Opaque types cater to the concept of the abstract data type
and information hiding. They are eliminated because again
the concept is covered by the new facility of extended
record types.
Enumeration types appear to be a simple enough feature to be
uncontroversial. However, they defy extensibility over
module boundaries. Either a facility to extend enumeration
types would have to be introduced, or they would have to be
dropped. A reason in favour of the latter, radical solution
was the observation that in a growing number of programs
the indiscriminate use of enumerations had led to a pompous
style that contributed not to program clarity, but rather
to verbosity. In connection with import and export,
enumerations gave rise to the exceptional rule that import
of a type identifier also causes the (automatic) import of
all associated constant identifiers. This exceptional rule
defies conceptual simplicity and causes unpleasant problems
for the implementor.
Subrange types were introduced in Pascal (and adopted in
Modula) for two reasons: (1) to indicate that a variable
accepts a limited range of values of the base type and
allow a compiler to generate appropriate guards for
assignments, and (2) to allow a compiler to allocate the
minimal storage space needed to store values of the
indicated subrange. This appeared desirable in connection
with packed records. Very few implementations have taken
advantage of this space saving facility, because additional
compiler complexity is very considerable. Reason 1 alone,
however, did not appear to provide sufficient justification
to retain the subrange facility in Oberon.
With the absence of enumeration and subrange types, the
general possibility to define set types based on given
element types appeared as redundant. Instead, a single,
basic type SET is introduced, whose values are sets of
integers from 0 to an implementation-defined maximum.
The basic type CARDINAL had been introduced in Modula-2 in
order to allow address arithmetic with values from 0 to
$2^{16}$ on 16-bit computers. With the prevalence of 32-bit
addresses in modern processors, the need for unsigned
arithmetic has practically vanished, and therefore the type
CARDINAL has been eliminated. With it, the bothersome
incompatibilities of operands of types CARDINAL and INTEGER
have disappeared.
The notion of a definable index type of arrays has also been
abandoned: All indecies are by default integers.
Furthermore, the lower bound is fixed to 0; array
declarations specify a number of elements (length) rather
than a pair of bounds. This break with a long standing
tradition since Algol 60 demonstrates the principle of
eliminating the inessential most clearly. The specification
of an arbitrary lower bound provides no expressive power at
all, but it introduces a non-negligible amount of hidden,
computational effort. (Only in the case of static
declarations can it be delegated to the compiler).
\subsection*{Modules and import/export rules}
Experience with Modula over the last eight years has shown
that local modules were rarely used. The additional
complexity of the compiler required to handle them, and the
additional complications in the visibility rules of the
language definition appear not to justify local modules.
The qualification of an imported object's identifier x by
the exporting module's name M, viz. M.x can be circumvented
in Modula by the use of the import clause FROM M IMPORT x.
This facility has also been discarded. Experience in
programming systems involving many modules has taught that
the explicit qualification of each occurrence of x is
actually preferable. A simplification of the compiler is a
welcome side-effect.
The dual role of the main module in Modula is conceptually
confusing. It constitutes a module in the sense of a
package of data and procedures enclosed by a scope of
visibility, and at the same time it constitutes a single
procedure called the main program. Within the Oberon system,
the notion of a main program has vanished. Instead, the
system allows the user to activate any (exported,
parameterless) procedure (called a command). Hence, the
language excludes modules without explicit definition parts,
and every module is defined in terms of a definition part
and an implementation part (not definition module and
implementation module).
\subsection*{Statements}
The with statement has been discarded. Like in the case of
exported identifiers, the explicit qualification of field
identifiers is to be preferred.
The elimination of the for statement constitutes a break
with another long standing tradition. The baroque mechanism
in Algol 60's for statement had been trimmed considerably
in Pascal (and Modula). Its marginal value in practice has
led to its absence in Oberon.
\subsection*{Low-level facilities}
Modula-2 makes access to machine-specific facilities
possible trough low-level constructs, such as the data
types ADDRESS and WORD, absolute addressing of variables,
and type casting functions. Most of them are packaged in a
module called SYSTEM. The features were supposed to rarely
used and easily visible trough the presence of SYSTEM in a
module's import list. Experience has revealed, however,
that a significant number of programmers import this module
quite indiscriminately. A particulary seducing trap are
Modula's type transfer functions.
It appears preferable to drop the pretense of portability of
programs that import a ``standard'', yet system-specific
module. Both the module SYSTEM and the type transfer
functions are eliminated, and with them also the types
ADDRESS and WORD. Individual implementors are free to
provide system-dependent modules, but they do not belong to
the general language definition. Their use then declares a
program to be patently implementation-specific, and thereby
non-portable.
\subsection*{Concurrency}
The system Oberon does not require any language facilities
for expressing concurrent processes. The pertinent,
rudimentary features of Modula, in particular the
coroutine, were therefore not retained. This exclusion is
merely a reflection of our actual needs within the concrete
project, but not on the general relevance of concurrency in
programming.
\section*{Features introduced in Oberon}
\subsection*{Type extension}
The most important addition is the facility of extended
record types. It permits the construction of new types on
the basis of existing types, and establishing a certain
degree of compatibility between the names of the new and
old types. Assuming a given type
\begin{verbatim}
T = RECORD x, y: INTEGER END
\end{verbatim}
extensions may be defined which contain certain fields in
addition to the existing ones. For example
\begin{verbatim}
T0 = RECORD (T) z: REAL END;
T1 = RECORD (T) w: LONGREAL END;
\end{verbatim}
define types with fields x, y, z and x, y, w respectively.
We define a type declared by
\begin{verbatim}
T' = RECORD (T) <field definitions> END
\end{verbatim}
to be a (direct) extension of T, and conversely T to be the
(direct) base type of T'. Extended types may be extended
again, giving rise to the following definitions:
A type T' is an extension of T, if T' = T or T' is a direct
extension of an extension of T. Conversely, T is a base of
T', if T = T' or T is the direct base type of a base type
of T'. We denote this relationship by T' {\tt =>} T.
The rule of assignment compatibility states that values of
an extended type are assignable to variables of their base
types. For example, a record of type T0 can be assigned to
a variable of the base type T. This assignment involves the
fields x and y only, and in fact constitutes a projection of
the value onto the space spanned by the base type.
It is important that an extended type may be declared in a
module that imports the base type. In fact, this is
probably the normal case.
This concept of extensible data type gains importance when
extended to pointers. It is appropriate to say that a
pointer type P' bound to T' extends a pointer type P, if P
is bound to a base type T of T', and to extend the
assignment rule to cover this case. It is now possible to
form structures whose nodes are of different types, i.e.
inhomogenious data structures. The inhomogeneity is
automatically (and most sensibly) bounded by the fact that
the nodes are linked by pointers of a common base type.
Typically, the pointer fields establishing the structure are
contained in the base type T, and the procedures
manipulating the structure are defined in the same (base)
module as T. Individual extensions (variants) are defined in
client modules together with procedures operating on nodes
of the extended type. This scheme is in full accordance
with the notion of system extensibility: new modules
defining new extensions may be added to a system without
requiring a change of the base modules, not even their
recompilation.
As access to an individual node via a pointer bound to a
base type provides a projected view of the node data only,
a facility to widen the view is necessary. It depends on
the possibility to determine the actual type of the
referenced node. This is achieved by a type test, a Boolean
expression of the form
\begin{verbatim}
t IS T' (or p IS P')
\end{verbatim}
If the test is affirmative, an assignment t' := t (t' of
type T') or p' := p (p' of type P') should be possible. The
static view of types, however, prohibits this. Note that
both assignments violate the rule of assignment
compatibility. The desired statement is made possible by
providing a type guard of the form
\begin{verbatim}
t' := t(T) (p' := p(P))
\end{verbatim}
and by the same token access to the field z of a T0 (see
previous examples) is made possible by a type guard in the
designator t(T0).z. Here the guard asserts that t is
(currently) of type T0.
The declaration of extended record types, the type test, and
the type guard are the only additional features introduced
in this context. A more extensive discussion is provided in
[2]. The concept is very similar to the class notion of
Simula 67 [3], Smalltalk [4], and others. Differences lie in
the fact that the class facility stipulates that all
procedures applicable to objects of the class are defined
together with the data declaration. It is awkward to be
obliged to define a new class solely because a method
(procedure) has been added or changed. In Oberon, procedure
(method) types rather than methods are connected with
objects in the program text. The binding of actual methods
(specific procedures) to objects (instances) is delayed
until the program is executed. In Smalltalk, the
compatibility rules between a class and its subclasses are
confined to pointers, thereby intertwining the concept of
access method and data type in an undesirable way. Here,
the relationship between a type an its extensions is based
on the established mathematical concept of projection.
In Modula, it is possible to declare a pointer type within
an implementation module, and to export it as an opaque
type by listing the same identifier in the corresponding
definition module. The net effect is that the type is
exported whereby its associated binding remains hidden
(invisible to clients). In Oberon, this facility is
generalized in the following way: Let a record type be
defined in a certain implementation part, for example:
\begin{verbatim}
Viewer = RECORD width, height: INTEGER; x, y: INTEGER END
\end{verbatim}
In the corresponding definition part, a partial definition
of the same type may be specified, for example
\begin{verbatim}
Viewer = RECORD width, height: INTEGER END
\end{verbatim}
with the effect that a partial view --- a public projection ---
is visible to clients. In client modules as well as in the
implementation part it is possible to define extensions of
the base type (e.g. TextViewers or GraphViewers).
\subsection*{Type inclusion}
Modern processors feature arithmetic operations on several
number formats. It is desirable to have all these formats
reflected in the language as basic types. Oberon features
five of them:
\begin{verbatim}
LONGINT, INTEGER, SHORTINT(integer types)
LONGREAL, REAL(real types)
\end{verbatim}
With the proliferation of basic types, a relaxation of
compatibility rules between them becomes almost mandatory.
(Note that in Modula the arithmetic types INTEGER, CARDINAL
and REAL are uncompatible). To this end, the notion of type
inclusion is introduced: a type T includes a type T', if the
values of T' are also values of type T. Oberon postulates
the following hierarchy:
\begin{verbatim}
LONGREAL > REAL > LONGINT > INTEGER > SHORTINT
\end{verbatim}
[Note that ``{\tt >}'' should be replaced by the appropriate
mathematical sign. Limitation of type-in..]
The assignment rule is relaxed accordingly: A value of type
T' can be assigned to a variable of type T, if T' is
included in T (if T' extends T), i.e. if T {\tt >} T' or T' {\tt =>} T.
In this respect, we return to (and extend) the flexibility
of Algol 60. For example, given variables
\begin{verbatim}
i: INTEGER; k: LONGINT; x: REAL
\end{verbatim}
the assignments
\begin{verbatim}
k:=i; x:=k; x:=1; k:=k+1; x := x*10 + i
\end{verbatim}
are confirming to the rules, where the assignments
\begin{verbatim}
i:=k; k:=x
\end{verbatim}
are not acceptable. Finally, it is worth noting that the
various arithmetic types represent a limited set of
subrange types.
The multi-dimensional open array and the closure statement
(in symmetry to a module's initialization body) are the
remaining facilities of Oberon not present in Modula.
\section*{Summary}
The language Oberon has evolved from Modula-2 and
incorporates the experiences of many years of programming
in Modula. A significant number of features have been
eliminated. They appear to have contributed more to
language and compiler complexity than to genuine power and
flexibility of expression. A small number of features have
been added, the most significant one being the concept of
type extension.
The evolution of a new language that is smaller, yet more
powerful than its ancestor is contrary to common practices
and trends, but has inestimable advantages. Apart from
simpler compilers, it results in a concise definition
document [5], and indispensible prerequisite for any tool
that must serve in the construction of sophisticated and
reliable systems.
\section*{Acknowledgement}
It is impossible to explicitly acknowledge all contributions
of ideas that ultimately simmered down to what is now
Oberon. Most came from the use or study of existing
languages, such as Modula-2, Ada, Smalltalk, C++ and Cedar,
which often though us how not to do it. Of particular value
was the contribution of Oberon's first user, J. Gutknecht.
The author is grateful for his insistence on the
elimination of deadwood and on basing the remaining
features on a sound mathematical foundation.
\section*{References}
\noindent
1. N. Wirth. Programming in Modula-2.
Springer-Verlag, 1982.
\noindent
2. N. Wirth. Type Extensions. ACM Trans. on Prog.
Languages and Systems (to appear)
\noindent
3. G. Birtwistle, et al. Simula Begin. Auervach,
1973.
\noindent
4. A. Goldberg, D. Robson. Smalltalk-80: The language
and its implementation. Addison-Wesley, 1983.
\noindent
5. N. Wirth. The Programming language Oberon
(language definition document)
\end{document}
--
Lawrence Crowl 716-275-9499 University of Rochester
crowl@cs.rochester.edu Computer Science Department
...!{allegra,decvax,rutgers}!rochester!crowl Rochester, New York, 14627kent@xanth.cs.odu.edu (Kent Paul Dolan) (02/29/88)
Moderately enraged response; add salt! The definition of Oberon looks, in the main, like a win, but somehow the baby went out with the bathwater! After a couple decades of using integers to encode concepts in FORTRAN and many other tongues, enumerated types were a gift from the dieties of programming; they rid the code of magic numbers, made it more readable and maintainable, and lessened dramatically the chance of coding errors. Since the automatic import of a type and all its named constants was already eliminated for Oberon, the explicit qualification of imported objects should have removed any implementation problems here. That is, if module A defines enumerated type a with constants p and q, and module B extends this to type b with additional constants r and s, and module C extends b to type c with added constant names t and u, surely in module C I can say: A.a.p A.a.q B.b.r B.b.s C.c.t, c.t or t C.c.u, c.u or u without any ambiguities? Where was the compelling need to eliminate this very valuable feature? Note that I assume here that we do NOT allow renaming, so that B must make the definitions imported from A visible to C, if C is to import them from B, and that this is exactly textually equivalent to C importing them from A directly. I really, really, really don't want to backslide to programs full of anonymous small integers which are really not arithmetic integers at all, but only cryptic encodings for logical problem space concepts which should have names, and the C language #define alternative makes me quite nauseous with all the #include conflicts that arise in trying to use this facility. Also, as another issue, if words, bitsets, and similar implementation hardware explicit concepts leave the standard language, then in the field of operating systems design, I would appreciate strengthening of the definition of sets, in the ways most implementations of Modula-2 have chosen to extend the language. Also, some language mechanism should (IMHO) be added to indicate that a piece of storage should be extended to the next boundary for convenient hardware access (i.e., if I define a set type of a size to require 7 bits, I should be able to force instances of elements of its powerset to extend to a multiple of the size of a character, short integer, integer, long integer, or pointer for execution efficiency, without being too bothered about just how big any of the latter items are in this particular hardware. I'm sure others would be just as interested in the opposite facility, to pack the elements for storage efficiency. Kent, the (just an old coder, no language design expert) man from xanth.
franka@mmintl.UUCP (Frank Adams) (03/01/88)
In article <7161@sol.ARPA> crowl@cs.rochester.edu (Lawrence Crowl) writes: >[Text by Niklaus Wirth] >With the proliferation of basic types, a relaxation of compatibility rules >between them becomes almost mandatory. To this end, the notion of type >inclusion is introduced: a type T includes a type T', if the values of T' >are also values of type T. Oberon postulates the following hierarchy: > LONGREAL > REAL > LONGINT > INTEGER > SHORTINT This appears to be a mistake. I am assuming that these are intended to be 64 and 32 bit reals, and 32, (16 or 32), and 16 bit integers, respectively. The basic problem is that the values of a 32 bit real do not include the values of a 32 bit integer. If one is going to be meticulous enough about assignments to define type inclusion, one should only make types be included when they really are. (If INTEGER is intended to be 32 bits, and LONGINT 64, the problems are even more serious. If REAL is intended to be a 64 bit float, the absence of a 32 float is a problem.) -- Frank Adams ihnp4!philabs!pwa-b!mmintl!franka Ashton-Tate 52 Oakland Ave North E. Hartford, CT 06108
sommar@enea.se (Erland Sommarskog) (03/02/88)
First, Lawrence, thank you for posting Wirth's paper. It was
interesting to read.
So Wirth has designed a new language. If he continues in this
direction he's going to end up with C next time :-) That is
to say I am not impressed. OK, the extended type facility is
a nice feature, but principally it's just another way of
expressing prefix classes, well-known from Simula. A somewhat
different approach is Ada's generic packages with private-
type parameters.
He seems to have got simplifications in the compiler on his
mind, totally ignoring that a simplification in the compiler may
end up as more work for the poor programmer. Of the restrictions
he has made, I'd like to particulary question four of them:
enumeration types, subranges, array indexes and for-statements.
Kent Paul Dolan has already given good arguments for keeping
enumeration types, so I won't reapeat , jsut strongly agree. However,
I don't share Kent's ideas on how to refer to imported enumration
values. That would lead to code like:
Case Action of
Module1.Action_type.Action1 : Do_something;
Module1.Action_type.Action2 : Do_something_else;
...
Talk about meaningless verbosity!
Wirth's argument for removing subranges is really short-cutting.
No one uses packing, and just guarding isn't sufficient enough,
he says in essence. Well, it is. Not the least when you remove
enumeration. If you had subranges, you could at least protect your
now integer-coded enumrates from illegal values due to assigning
from uninitiated variables.
I don't fully understand the passage on arrays. He says that indexes
are integers "by default". Does this mean you could have a character
index if you like? The rest of the passage doesn't seem to admit it.
And that is a miss, I think.
The idea of having all arrays starting on index zero is really to
put the burden on the programmer. If Wirth really must have a fixed
lower index, he could at least have chosen one! How many of you do
often use arrays with zero as the lower bound? I do it very seldom.
And his argument about saving execution time for index computation
is truely naive. The result will just be references like:
String(char_no + 1),
just making the code more verbose and complex for no use.
The removal of the FOR-statement is also questionable. This means
that all loops must be done with WHILE (or REPEAT?). WHILE is more
error-prone than FOR, since there are more things that the programmer
has to take care of.
And particulary in Pascal, and I guess also Modula, a certain mistake
may have disastrous consequences:
PROCEDURE A;
VAR i : integer;
PROCEDURE B;
BEGIN
FOR i := 1 TO Max DO
Something;
END;
BEGIN
FOR i := 1 TO Max DO
B;
END;
Since we forgot to declare i in B, B will be called only once. (OK,
depends on how FOR is implemented, but replace with WHILE and there is
no doubt.) Most Pascal implementions help to save you from this by
allowing an arbitrary order of the declarations, so we can desclare i
just before A's body. (How about Modula and Oberon?)
Here I can't keep from mentioning Ada. In Ada the loop variable is
declared in the FOR statement as such. Now, if only things like
FOR i IN Array_range WHILE A(i) /= 0 LOOP
had been allowed...
Wirth also talks about the importance of the description being concise.
My doubts here. OK, "Programming in Modula-2" may not count as the
description, but I recall it as truely drivelling in many parts. (Also,
reading it directly after going through the Ada RM, was like drinking
diluted lemonade.) And his code examples were about unreadable, on top
of all being set with a proportional font!
We had an discussion on "Modern languages" a while ago, me being guilty.
A current trend in language design seem to the removal of long-ago
accpted constructs. Oberon drops enumerations, Eiffel has no arrays.
The only difference is that while Oberon leaves no substitions except
integer constants, Eiffel provides arrays as a standard class. Also,
Eiffel, does really introduce something new: Multiple inheritence.
Missing in many OO-languages as Simula, Smalltalk and also Oberon.
--
Erland Sommarskog
ENEA Data, Stockholm
sommar@enea.UUCP "Souvent pour s'amuser les hommes d'equipages
and it's like talking to a stranger" -- H&C.crowl@cs.rochester.edu (Lawrence Crowl) (03/03/88)
In article <2787@enea.se> sommar@enea.UUCP(Erland Sommarskog) writes: >The extended type facility is a nice feature, but principally it's just >another way of expressing prefix classes, well-known from Simula. A somewhat >different approach is Ada's generic packages with private-type parameters. Ada's generic packages may introduce a new procedure to handle each parameter type. Oberon's extended types guarantee that exactly one procedure is needed. Ada's generic packages allow only one type to be in the corresponding structure (e.g. queue), while Oberon's extended types allow many different types to be in the queue. Ada's generic packages allow putting a type on a queue to be done simply as an afterthought, but using Oberon's extended types requires the more effort to put a type on a queue as an afterthought. >The idea of having all arrays starting on index zero is really to put the >burden on the programmer. ... And his argument about saving execution time >for index computation is truely naive. The result will just be references >like: "String(char_no + 1)", just making the code more verbose and complex >for no use. One compiler technique adjusts the "base" address portion of the index computation to account for non-zero lower element. This optimization becomes harder if non-zero basing is done in user code. -- Lawrence Crowl 716-275-9499 University of Rochester crowl@cs.rochester.edu Computer Science Department ...!{allegra,decvax,rutgers}!rochester!crowl Rochester, New York, 14627
djsalomon@watdragon.waterloo.edu (Daniel J. Salomon) (03/04/88)
I would like to point out two minor typo's in Wirth's introduction
to Oberon that lead to humorous results. On page 3 he gives statistics
on code size for the parts of his Oberon compiler. The correct table
should be:
length of length of time of self
source program compiled code compilation
----------------------------------------------------------------
lines characters bytes seconds
----------------------------------------------------------------
Parser 1116 36719 9928 11.53
Scanner 346 9863 3388 3.80
Import/Export 514 18386 4668 5.25
Code generator 1963 65901 21636 21.02
------ ------- ------- -------
Total 3939 130869 39620 41.60
Notice that only two digits have changed columns, but now the columns
add up correctly, and the statistics make more sense.nevin1@ihlpf.ATT.COM (00704a-Liber) (03/04/88)
In article <2740@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes: .In article <7161@sol.ARPA> crowl@cs.rochester.edu (Lawrence Crowl) writes: .>[Text by Niklaus Wirth] .>With the proliferation of basic types, a relaxation of compatibility rules .>between them becomes almost mandatory. To this end, the notion of type .>inclusion is introduced: a type T includes a type T', if the values of T' .>are also values of type T. Oberon postulates the following hierarchy: .> LONGREAL > REAL > LONGINT > INTEGER > SHORTINT . .The basic problem is that the values of a 32 bit real do not include the .values of a 32 bit integer. If one is going to be meticulous enough about .assignments to define type inclusion, one should only make types be included .when they really are. This depends on what is meant by value. Including precision, you are right; n bit reals cannot exactly represent all the n bit integers. But, if you allow the precision to 'slack off', n bit reals CAN represent all the n bit integers. But the mapping is not 1:1; it is many:1 (from ints to reals); this means that in the following case: VAR IX, IY : INTEGER; {integers are 32 bits} RZ : REAL; {reals are 32 bits} ... {Somethings sets IX} ... RZ := IX; IY := INTEGER(RZ); IX and IY are not (necessarily) equal. But I wonder: in this circumstance, is it possible for the IY assignment statement to ever fail (ie, produce a run-time error)? I would hope not. A lot of people have praised Pascal and Modula-2 BECAUSE there were no implicit type conversions. Me, I'm not sure whether this is good or bad. Personally, I make most of my type conversions explicit anyway in most of the languages I use. Does it really add all that much complexity to the compiler; or, more importantly, does the compiler generate better code if the conversions are implicit (Personally, I can't see this happening)? -- _ __ NEVIN J. LIBER ..!ihnp4!ihlpf!nevin1 (312) 510-6194 ' ) ) "The secret compartment of my ring I fill / / _ , __o ____ with an Underdog super-energy pill." / (_</_\/ <__/ / <_ These are solely MY opinions, not AT&T's, blah blah blah
oconnor@sunset.steinmetz (Dennis M. O'Connor) (03/05/88)
It's difficult to see the logic in using Modula2 if
a reliable Ada(R) compiler is available. This is
precisely my situation : I use Modula2 as a "poor
man's Ada", on my Amiga. Not that it isn't okay,
but Ada leaves Modula2 far behind.
Oberon seems to be a "poor man's Modula2", which
I guess makes it a "welfare Ada" :-) I see no
use for Oberon. With the ever-increasing availablity
of "standard" C, Ada, Common Lisp, and yes Modula2,
it has no unique problem domain in which it excels
by enough of a margin to make it worth the hassle
that is ALWAYS involved in using a new language.
Sad what can happen when famous intellectuals try
to recapture their glory days, isn't it :-)
--
Dennis O'Connor oconnor%sungod@steinmetz.UUCP
ARPA: OCONNORDM@ge-crd.arpa
(-: The Few, The Proud, The Architects of the RPM40 40MIPS CMOS Micro :-)cjeffery@arizona.edu (Clinton Jeffery) (03/07/88)
From article <9796@steinmetz.steinmetz.UUCP>, by oconnor@sunset.steinmetz (Dennis M. O'Connor): > It's difficult to see the logic in using Modula2 if > a reliable Ada(R) compiler is available...[verbiage omitted]... > Oberon seems to be a "poor man's Modula2", which > I guess makes it a "welfare Ada" :-) I see no use for Oberon...[verbiage]... > it has no unique problem domain in which it excels ***FLAME ON*** Bashing Wirth without understanding his point is silly, especially by contrasting his work with Ada! I may not be excited about Oberon, because I am particularly fond of FOR loops :-), but it is more sensible to design a general purpose language that accomodates abstractions efficiently than to try to provide ALL abstractions as builtins! Mr. O'Connors last point about unique problem domains is VERY apt, but applies equally well to that thing people call Ada. Get Ada lovers out of this newsgroup; they don't understand what Modula-2 and friends are for. -- +-------------- | Clint Jeffery, University of Arizona Department of Computer Science | cjeffery@arizona.edu -or- {ihnp4 noao}!arizona!cjeffery +--------------
oconnor@sungoddess.steinmetz (Dennis M. O'Connor) (03/09/88)
An article by cjeffery@arizona.edu (Clinton Jeffery) says:
] From article <9796@steinmetz.steinmetz.UUCP>, by oconnor@sunset.steinmetz (Dennis M. O'Connor):
] > It's difficult to see the logic in using Modula2 if
] > a reliable Ada(R) compiler is available...
] > Oberon seems to be a "poor man's Modula2", which
] > I guess makes it a "welfare Ada" :-) I see no use for Oberon...
] > it has no unique problem domain in which it excels
]
] ***FLAME ON***
] Bashing Wirth without understanding his point is silly, especially by
] contrasting his work with Ada!
As I understand it, the point of Oberon is to make the compiler writer
do LESS, and the application programmer do more. Which, if true, is INSANE.
] [...] it is more sensible to design
] a general purpose language that accomodates abstractions efficiently than
] to try to provide ALL abstractions as builtins!
If there's a language that does "provide ALL abstractions as
builtins", it is NOT Ada. Ada is VERY extensible, with its
private types, limited types, packages and generics. What the
HELL are you talking about ? Have you programmed in Ada ??
Have you even read the LRM ? Nice book, that LRM.
] Mr. O'Connors last point about unique problem domains is VERY apt,
] but applies equally well to that thing people call Ada.
Sorry, Ada has a unique problem domain : software for the military.
Due to legislation more than anything else, perhaps. But you are
still wrong. IMHO, Ada is a VERY good language for LARGE SYSTEM
development by MANY programmers. That's a BIG problem domain.
] Get Ada lovers out of this newsgroup; they don't
] understand what Modula-2 and friends are for.
My, my, aren't we opinionated? "Modula-2 and friends"?? That's a
funny statement, since Ada and Modula-2 are practically sisters.
Now me, I've programmed EXTENSIVELY (not just homework) in
ZetaLISP, C, Ada, Modula-2, FORTRAN, and assembly. I like
Ada, I like Modula-2, I like Zetalisp. I love my wife.
( I HATE CASE SENSITIVITY IN A LANGUAGE, tho )
Have you written large code in Ada ?? If not, you have no
right to talk about it. So SHUT UP.
] | Clint Jeffery, University of Arizona Department of Computer Science
Boy, the things these college students say...
--
Dennis O'Connor oconnor%sungod@steinmetz.UUCP
ARPA: OCONNORDM@ge-crd.arpa
(-: The Few, The Proud, The Architects of the RPM40 40MIPS CMOS Micro :-)snorri@rhi.hi.is (Snorri Agnarsson) (03/09/88)
In my opinion Oberon is a significant improvement over Pascal, Modula-2 and Ada in one respect: It has automatic garbage collection. This feature is enough to choose Oberon over the other three. I would rather have list processing without modules than modules with no list processing. Of course having both is best. -- Snorri Agnarsson Internet: snorri@rhi.hi.is Raunvisindastofnun Haskolans uucp: ...!mcvax!hafro!krafla!snorri Dunhaga 3 107 Reykjavik ICELAND
cbseeh@fang.ATT.COM (Edmund E Howland) (03/14/88)
> > In my opinion Oberon is a significant improvement over > Pascal, Modula-2 and Ada in one respect: It has > automatic garbage collection. This feature > is enough to choose Oberon over the other three. > I would rather have list processing without modules than > modules with no list processing. Of course having both > is best. > -- Huh? Since when have these three ever had a need for garbage collection? I am not too familiar with Ada, but garbage collection exists in languages for whom dynamic binding is a way of life, not an option. It seems you really are saying that you like languages with imbedded list processing. Modula-2, Pascal and Ada do not directly support list contructs in their syntax, but it is of course possible to roll-your-own, so to speak. Garbage collection is not really the issue. It all boils down to the right tool for the right job. I think it unwise to pick a language over another because of the lack of some feature, in the latter. I might argue that Ada is a better language than Modula 2, simply because it has better concurancy primitives. But, if the application at hand had no use for these features, my argument would be invalid. Indeed, Modula 2 would be the better choice since the implementation would be smaller, resulting in tighter code, and in the long run more maintainable.
dhesi@bsu-cs.UUCP (Rahul Dhesi) (03/16/88)
In article <272@fang.ATT.COM> cbseeh@fang.ATT.COM (Edmund E Howland) writes: >I might argue that Ada is a better language >than Modula 2, simply because it has better concurancy primitives. But, if >the application at hand had no use for these features, my argument would be >invalid. Indeed, Modula 2 would be the better choice since the implementation >would be smaller, resulting in tighter code, and in the long run more >maintainable. If your do not need any concurrency in your application, a smart Ada compiler will generate code not containing any provision for concurrency. Therefore there is no reason to believe that the code generated from the Modula-2 program will be smaller. -- Rahul Dhesi UUCP: <backbones>!{iuvax,pur-ee,uunet}!bsu-cs!dhesi
snorri@rhi.hi.is (Snorri Agnarsson) (03/16/88)
From article <272@fang.ATT.COM>, by cbseeh@fang.ATT.COM (Edmund E Howland): > It seems you really are saying that you like languages with imbedded > list processing. Modula-2, Pascal and Ada do not directly support > list contructs in their syntax, but it is of course possible to roll-your-own, > so to speak. Garbage collection is not really the issue. It all boils > down to the right tool for the right job. > Not true. It is not a matter of syntax and it is not possible in general to "roll-your-own" in a satisfactory manner. To really support abstraction garbage collection is a necessity. If you don't believe me, try to write list processing primitives in MODULA-2 that work without their user having to explicitly deallocate unused space and allow the user to write list processing constructs in a reasonable way, i.e.: X := CONS(Y,Z); X := HEAD(Y); X := TAIL(Y); It can't be done in any MODULA-2 or PASCAL version I know of. Ada compilers can theoretically be written in such a way that garbage collection is possible (at least so I'm told), but I don't know if any such compilers exist. It does not seem to be considered a big issue. In my opinion it is. -- Snorri Agnarsson Internet: snorri@rhi.hi.is Raunvisindastofnun Haskolans uucp: ...!mcvax!hafro!krafla!snorri Dunhaga 3 107 Reykjavik ICELAND
cornwell@NRL-CSS.ARPA (Mark Cornwell) (03/19/88)
>I might argue that Ada is a better language >than Modula 2, simply because it has better concurancy primitives. But, if >the application at hand had no use for these features, my argument would be >invalid. Indeed, Modula 2 would be the better choice since the implementation >would be smaller, resulting in tighter code, and in the long run more >maintainable. If your do not need any concurrency in your application, a smart Ada compiler will generate code not containing any provision for concurrency. Therefore there is no reason to believe that the code generated from the Modula-2 program will be smaller. -- Rahul Dhesi UUCP: <backbones>!{iuvax,pur-ee,uunet}!bsu-cs!dhesi # Personally, I tend to distrust approaches that depend on the compiler # being "smart". Typically compilers aren't. E.g. it is widely know that # a smart compiler can implement tail-recursion with a tight loop and eliminate # the recursive procedure call. However, I only know of two compilers that # actually perform that optimization. # # Building an Ada compiler so that it passes the verification suite is # such a herculean task that I doubt seriously the compiler writers # have the resources to spare for clever optimizations. # # Also, I would suggest that the size and complexity of the # Ada language and its compilers could make many usually feasible # optimizations much more difficult. I have been told by those in the # trenches writing Ada compilers that they are orders of # magnitude more complex and difficult to write than the compilers for # simpler and more elegant languages such as Algol, C, Pascal, and Modula. # # # Regarding 0/1-based arrays: Trust me, 0-based is better. Just get # used to saying "a collection of N elements, e[0],...,e[N-1]" when # stating the problem and you'll find it makes the world simpler. # (You can also say: "a collection of N+1 elements, e[0],...,e[N], if you like.) # # I don't advise switching back and forth between 1-based and 0-based, you'll make # mistakes. I have my theories as to why 0-based leads to simpler programs, # but I won't state them here. I do find that the best programmers and # mathematicians I know seem to agree with me that 0-based indexing simply # works better in practice. I think its the sort of issue where only personal # experience can convince. To be convinced, you have to try it. # # I understand that some of Galileo's contemporaries refused to look through his # telescope so they could maintain their position that Jupiter had no moons. # Now, with the renaissance at hand, who wants to program in the middle ages? # #----------------------------------------------------------------------------- # Mark Cornwell cornwell@nrl-css.arpa # Naval Research Laboratory # # 0,..N-1: the right way ;)
fortin@zap.UUCP (Denis Fortin) (03/29/88)
In article <272@fang.ATT.COM> cbseeh@fang.ATT.COM (Edmund E Howland) writes: >>> [talking about the definition of the Oberon language posted recently >>> to comp.lang.modula2] >> In my opinion Oberon is a significant improvement over Pascal, Modula-2 >> and Ada in one respect: It has automatic garbage collection. This feature >> is enough to choose Oberon over the other three. > >Huh? >Since when have these three ever had a need for garbage collection? >I am not too familiar with Ada, but garbage collection exists in >languages for whom dynamic binding is a way of life, not an option. (note: the stuff below applies equally well to Modula-2 or Ada, although it is a bit more Ada-specific) Actually, Ada *allows* but does not not *require* the existence of a garbage collector (LRM section 4.8 paragraph 7). While I agree that for real-time embedded applications, a garbage collector can be annoying (though there are incremental algorithms that make this less true), in a language that encourages information-hiding, a good garbage collector is almost a must (in my opinion). The problem is that users might be using modules/packages that contain hidden pointers to dynamic data. Without garbage collection, you have to have a free/dispose/unchecked_deallocation routine available for all hermetic data types you export, otherwise code will be written that does not release variables of that type after using them and if at a latter point you change the implementation of the hidden data type (which you SHOULD be able to do, that's why you hid it's true contents in the first place) and stick a pointer/access variable in it, then you're stuck -> your heap will slowly erode away! Actually, by not requiring that a user call a "free" routine for all variables of data types you export (those where implementation is hidden), you are allowing him/her to make a very important assumption about the way your data type is implemented (which is bad). Of course, one CAN export a "free" routine with *ALL* data types, but then it becomes a real pain to write code that uses the data type. For example, a "dynamic string package" would require that you call "free(...)" for all of the string variables you declare at the end of each block that declares such variables -> this is not only notationally painful (!?), but also very error-prone. Anyway, I've been involved in a 75000+ lines Ada project, and every time I can talk to Ada compiler vendors, I always ask them *when* garbage collection will finally be available for their compiler. (Put in a pragma to turn it off for real-time applications or something...) (Actually, the two biggest demands that my group had for Ada compilers (besides the obvious "fast compile" and "fast execution") where "fast tasking" and "garbage collection"!) Sigh, it felt good getting this off my chest :-) -- Denis Fortin | fortin@zap.UUCP CAE Electronics Ltd | philabs!micomvax!zap!fortin The opinions expressed above are my own | fortin%zap.uucp@uunet.uu.net
crowl@cs.rochester.edu (Lawrence Crowl) (03/30/88)
In article <429@zap.UUCP> fortin@zap.UUCP (Denis Fortin) writes: >... in a language that encourages information-hiding, a good garbage >collector is almost a must (in my opinion). ... Without garbage collection, >you have to have a free/dispose/unchecked_deallocation routine available for >all hermetic data types you export, ... [otherwise you limit subsequent >implementations] ... but then it becomes a real pain to write code that uses >the data type. C++ provides the notion of a *destructor*. Whenever you define a class (abstract data type), you may define a destructor. The destructor is *implicitly* called when the variable is deleted (e.g. at the end of the procedure for local variables). The implementation of the class may change from static allocation to dynamic allocation, without changing the client code, without leaking memory, and without explicit client control of deallocation. Garbage collection is one approach to reducing the storage management burden on programmers. It is not the only approach, nor is it always the best approach. -- Lawrence Crowl 716-275-9499 University of Rochester crowl@cs.rochester.edu Computer Science Department ...!{allegra,decvax,rutgers}!rochester!crowl Rochester, New York, 14627
boehm@titan.rice.edu (Hans Boehm) (03/31/88)
It seems to me that the C++ approach to storage management is at best
a partial solution. Admittedly it succeeds at automatically deallocating
objects in trivial cases. For some applications this is, no doubt,
sufficient. But consider a case as simple as implementing a stack type,
whose underlying representation is a linked list. Assume this type
includes a non-destructive "pop" operation that returns a new stack
one shorter than the old one. The original stack is left intact. ("Pop"
can of course be implemented as the "tail" operation on linked lists.)
Should the head of the original linked list be deallocated? Is it reasonable
to make that the caller's responsibility? Certainly not, since it's not
supposed to know anything about the representation of stacks. After all,
I could be copying arrays. The stack implementation could reference count,
but that's more tedious, error prone, and probably slower than a good
garbage collector. It also doesn't always work.
My experience is that any attempt at manipulation of interesting linked
structures in a language that doesn't support real automatic storage
management will either fail, or waste large amounts of debugging time.
(My experience includes a (working) 40,000 line compiler, written in C, that
manipulates a reference counted syntax "tree", or more accurately, a
reference counted syntax graph.) Normally, it is extremely difficult
to track down bugs created by premature deallocation. When such bugs are
finally removed, the resulting programs normally include a substantial
number of storage leaks.
Some recent experiments by Mark Weiser and myself with retrofitting a
garbage collector to existing C programs, verify the latter point.
(The garbage collector should never free anything since that was the
programmers responsibility. It does. In other people's code.
Our paper on this should appear in Software P&E shortly.)
Mike Caplinger reported similar results for another application at the last
USENIX conference, I believe. We have resurrected C code with storage
management bugs by linking it against a garbage collector (which in the case
of C doesn't always work either, but it has a better track record than manual
storage management).
There are arguably cases in which a garbage collector is undesirable,
notably in the presence of severe real-time constraints. But even in such
a situation, I would want a garbage collector during debugging to help
me track down storage leaks.
Hans-J. Boehm boehm@rice.edu
Computer Science Dept.
Rice University
Houston, TX 77251-1892