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Archive-name: Modula-3-faq
Michel Dagenais Michel Dagenais, GNU General Public License, 1998-2001
michel.dagenais@polymtl.ca
Ecole Polytechnique
C.P. 6079, Succ. Centre-Ville
Montreal, Quebec, H3C 3A7
Modula-3 Frequently asked questions and answers
Maintained by Michel Dagenais ([1]michel.dagenais@polymtl.ca),
suggestions are most welcome. Last updated January 15 2002. The latest
copy of this FAQ may be obtained from the [2]Polytechnique Montreal
Modula-3 Home.
Introduction
What is Modula-3?
Modula-3 is a systems programming language that descends from Mesa,
Modula-2, Cedar, and Modula-2+. It also resembles its cousins Object
Pascal, Oberon, and Euclid.
The goal of Modula-3 is to be as simple and safe as it can be while
meeting the needs of modern systems programmers. Instead of exploring
new features, we studied the features of the Modula family of
languages that have proven themselves in practice and tried to
simplify them into a harmonious language. We found that most of the
successful features were aimed at one of two main goals: greater
robustness, and a simpler, more systematic type system.
Modula-3 retains one of Modula-2's most successful features, the
provision for explicit interfaces between modules. It adds objects and
classes, exception handling, garbage collection, lightweight processes
(or threads), and the isolation of unsafe features.
Where is Modula-3 used? Is it used in industry?
A number of programming teams selected Modula-3 for industrial and
research projects, and for teaching. It encourages good programming
practices and comes with excellent libraries for distributed
programming and graphical user interfaces. A non exhaustive list is
available at [3][Modula-3 at Work].
Is commercial support available?
Critical Mass Corporation used to offer their own version of SRC
Modula-3, CM3, an integrated development environment for Modula-3,
[4]Reactor, as well as training and consulting services.
Olaf Wagner from [5]Elego Software Solutions is now maintaining [6]CM3
as an open source package and offers commercial support.
Where can I get documents and information on Modula-3?
A [7]concise bibliography and a more complete [8]bibliography describe
Modula-3 related books, technical reports, and papers. The definition
of Modula-3 is contained in: [9]"System Programming with Modula-3"
also known as SPwM3. Sam Harbison has written a more tutorial book
titled [10]Modula3.
Three main Web servers contain Modula-3 related information:
[11]www.m3.org, [12]DEC SRC Modula-3 home page, and [13]Ecole
Polytechnique de Montré Modula-3 home page.
The Usenet newsgroup comp.lang.modula3 is the official meeting place
for Modula-3 related discussions.
Is Modula-3 a superset of Modula-2?
No; valid Modula-2 programs are not valid Modula-3 programs. However,
there is a tool to help convert Modula-2 programs to Modula-3.
Comparisons between Modula-3 and other languages?
From: laszlo@post.ifi.uni-klu.ac.at (Laszlo BOESZOERMENYI)
A Comparison of Modula-3 and Oberon-2 by myself in Structured
Programming 1993, 14:15-22
From: nayeri@gte.com
Robert Henderson, Benjamin Zorn, A Comparison of Object-Oriented
Programming in Four Modern Languages, Department of Computer Science,
University of Colorado, Boulder, Colorado, [14]Technical Report
CU-CS-641-93.
The paper evaluates Oberon, Modula-3, Sather, and Self in the context
of object-oriented programming. While each of these programming
languages provide support for classes with inheritance, dynamic
dispatch, code reuse, and information hiding, they do so in very
different ways and with varying levels of efficiency and simplicity. A
single application was coded in each language and the experience
gained forms the foundation on which the subjective critique is based.
What implementations are available, how do they compare?
All implementations are based on [15]DEC SRC Modula-3. [16]Critical
Mass offered an improved version with commercial support. It features
incremental garbage collection on NT, and a few additional packages
like ODBC database access. This is now [17]open sourced and maintained
by Olaf Wagner. Ecole Polytechnique de Montréal has been maintaining
an [18]updated distribution. It features integrated documentation, and
NT support through the gcc cygwin compiler
Can I contribute Modula-3 software?
Contributions are most welcome. The primary contact to offer
contributions is comp.lang.modula3.
[19]The Ecole Polytechnique de Montréal Modula-3 distribution is the
most regularly updated and may be a good place to submit your
contribution.
Why use Modula-3?
Here is what John Polstra, author of the popular CVSup, replied:
Subject: Re: SUP on sup.freebsd.org
Date:Wed, 06 Nov 1996 12:31:26 -0800
From:John Polstra
> Erhm, why on earth did you chose Modula3 ??
Modula-3 really is a different language, designed specifically for
systems programming by some extremely competent and experienced people
who knew what they were doing.
> Oh and yes I have seen apps written in modula3, all of which was
> horrible performers, and impossible to port to new platforms, so
> the management decide a complete rewrite in, guess what, C!
Are you sure it was Modula-3? The SRC Modula-3 compiler supports about
25 different platforms.
Plenty of real world apps (*big* ones) have been written in Modula-3,
and they perform pretty well. There's also the SPIN OS project
[20][SPIN] at University of Washington, in which the kernel was
written in Modula-3. It performs well, too.
Now, you can always argue that a program would be somewhat faster and
somewhat smaller if it had been written in C. Hey, guess what? I was
around when Unix V6 came out, and the same stuff was written about it.
Just substitute "C" for "Modula-3" and "assembly language" for "C".
The answer is the same in both cases: Unix would not exist as we know
it today if it had been written in assembly language. CVSup would not
exist as we know it today if it had been written in C (or C++, for
that matter).
OK, so why on earth did I choose Modula-3? In no particular order:
1. I needed application level threads, and threads are an integral
part of the Modula-3 language. About the only reasonable
alternative was to use pthreads with C or C++. But pthreads was
not well supported under FreeBSD at that time.
2. I needed a graphical display during development so that I could
monitor the 3 client threads as they were running, debug them,
appraise their relative performance, and find the bottlenecks.
Modula-3 has a very nice toolkit for creating GUIs quickly and
painlessly. (OK, so the scrollbars are as ugly as sin.)
3. Modula-3 is a compiled language that is reasonably efficient.
4. I needed to use some low level system functions, e.g., mapping
files into memory. Modula-3 provides good access to such
functions, and it is quite easy to add interfaces to foreign
libraries such as "libz".
5. Modula-3 has good support for networking.
6. It is a mature and stable language that has been used in a number
of serious, large projects. The language and compiler have been
stable for about 5 years, which is more than you can say for C++.
7. It has nice support for object oriented programming, including a
good type system, a nice exception model, and a modern
high-performance garbage collector. These traits, IMHO, contribute
powerfully to producing well-structured, maintainable programs.
Now before you label me an unstudly OO weenie, please consider
this. I've been programming in C professionally for 19 years. I
made my living for many years writing C compilers and related
tools such as assemblers, linkers, and disassemblers. I still use
C and C++ when I feel they are appropriate for a project, not to
mention when I have to because that's what the client wants to
use. I have experience programming in many many different
languages. Different languages are good for different things. I
still like programming in C (and C++ for some things), but I'm
glad I didn't use it for CVSup.
8. I had just come off a huge 3+ year C++ project. During that time,
I learned just how much C++ sucks. I did not feel like doing it
again right away "for fun."
9. I have spent my entire professional career getting paid to use the
wrong tools, because, e.g., the manager read that C++ was
"popular." For once, just once, on a _hobby_ project, I decided I
was going to use the tool I felt was the best for the job at hand.
I thought about it long and hard, evaluated several options (C and
C++ among them), and eventually chose Modula-3. I have never
regretted that decision.
Any questions? :-)
John
- --
John Polstra jdp@polstra.com
John D. Polstra & Co., Inc. Seattle, Washington USA
"Self-knowledge is always bad news." -- John Barth
Troubleshooting
Why program receives a SEGV signal under the debugger?
The garbage collector on some platforms uses the SEGV (segmentation
violation) signal to detect modified portions of the dynamically
allocated space. It is possible to disable this feature or to inform
the debugger to let these signals propagate. See the [21]m3gdb
documentation.
Problems with threads, fork and VTALARM?
The threads are implemented using the virtual timer interrupt.
Normally, the run time environment will catch the interrupt and
determine if thread switching is appropriate. However, if a new
process is created with fork, it will have the virtual timer activated
and no interrupt handler to receive it, resulting in a core dump. If
you use the standard procedure Process.Create to fork new processes,
this will be handled automatically for you. If you insist on using
fork, you need to disable the timer, fork and then reenable the timer.
X libraries not found?
The position of X libraries is stored, for instance for pre-compiled
PM3 LINUXELF binaries, in the template file m3config/src/LINUXELF as
well as in X11/LINUXELF/.M3EXPORTS (m3build/templates/LINUXELF, and
X11R4/LINUXELF/.M3EXPORTS for SRC-M3). Thus you may want to edit these
files if your X libraries are located in an uncommon place.
What means Missing RTHooks or similar messages?
The standard library, libm3, is not included by default. You need in
your m3makefiles to import("libm3") or to import a library which
imports libm3. Otherwise, messages about run time procedures such as
RTHooks not being available are produced.
M3build versus Make or why m3 does not work?
The Modula-3 compiler m3 does a much finer grained dependency analysis
than possible with make. For this reason, a very flexible front end,
m3build, reads the program description files, m3makefile, and
generates the commands required to compile and link Modula-3 programs
and libraries. The m3makefile content is documented in the m3build
documentation. Calling the m3 compiler directly is difficult and thus
not recommended, especially on PM3 where it is now merged with
m3build.
Why are exceptions raised by X or Network Objects applications?
Graphical applications (based on Trestle/X Windows) raise the
TrestleComm.Failure exception when the DISPLAY environment variable is
incorrect or the X server is refusing the connection. They raise
MachineIDPosix.Failure if the network configuration files are
incorrectly set up, especially on LINUX; /etc/hosts must contain at
least a loopback address (127.0.0.1) and the /etc/rc scripts an
appropriate ifconfig command (/etc/ifconfig lo 127.0.0.1; /etc/route
add 127.0.0.1). Applications with Network Objects may also raise
exceptions or consume all the CPU time available when the network
configuration files are incorrect.
What is the story with Trestle and OpenWindows?
Mark Manasse says:
I think that the OpenWindows release should be enough (no need to get
the MIT X release), although there are a few things in Trestle that
trigger devastating bugs in OpenWindows. But the only library we
depend on is Xlib, R4 or later.
The main thing I know that crashes OW 2.0 is the code where we call
GrabKey specifying AnyKey. You can either loop over all of the keys,
or you can just comment out the call; programs won't run exactly the
same, but you probably won't notice the difference.
Why so many problems installing on Solaris?
These notes were contributed by (simon.hood@umist.ac.uk) while
installing PM3-1.1.14 on a Sun Ultra 5 running Solaris 2.8. They
describe various problems and their solution or workaround.
The installation of PM3 on Solaris systems is particularly prone to
problems as these systems tend to be an unpredictable (from the point
of view of the PM3 people) mixture of Sun and Gnu software --- Sun do
not bundle a C compiler with the operating system.
My machine has gcc version 2.95.2 installed; it has Sun's versions of
make, ld, as and ar installed within /usr/ccs/bin; Gnu's version of
these tools are not installed.
My installation was successful, after a bit of fiddling around with
the configuration/template files and environment variables. Some of
the fixes are trivial (if you know what you are doing), while others
--- for me at least --- were not --- I am a Modula 3 novice and far
from experienced with Solaris.
The issues that arose are:
* Paths needed to be set to find tools such as make.
* LD_LIBRARY_PATH needed to be set to ensure libstdc++.a.2.10.0
and/or libstdc++.so.2.10.0 were found.
* A link needed to be set so that byacc points to yacc.
* The configuration for linking needed to be changed since only the
Sun version of ld was installed, not Gnu's.
* The build of m3gdb failed to build.
* The gnuemacs package failed to build.
Initial Problems
Paths
gcc is usually installed in /usr/local/bin; on a Solaris
machine, ar, as, make and ld are all in /usr/ccs/bin, by
default. Hence these must both be on root's path (assuming you
are installing as root). Neither were; I have not changed any
paths since installation of Solaris 2.8 on a new machine a few
days ago.
This is in addition to /usr/local/pm3/bin, as mentioned by the
PM3 installation instructions.
byacc/yacc
The build required byacc. yacc is installed in /usr/ccs/bin; a
soft link:
lrwxrwxrwx 1 root other 4 Aug 11 15:45 byacc -> yacc
solved this problem.
Library Paths
In addition to /usr/local/pm3/lib/m3 as mentioned by the PM3
installation instructions LD_LIBRARY_PATH must include
/usr/local/lib so that libstdc++ (part of the gcc distribution)
can be found.
CC
In addition I found that the environment variable CC needed to
be set to /usr/local/bin/gcc. This is of course mentioned in
the PM3 installation instructions.
Miscellaneous Questions
Can I get Modula-3 other than by FTP or HTTP?
Prime Time Freeware (PTF) includes Modula-3. PTF is a set of two
ISO-9660 CDroms filled with 3GB of freeware, issued semi-annually. PTF
is distributed via bookstores and mail. You can reach PTF using:
Email: ptf@cfcl.com
Fax: [1] (408) 738 2050
Voice: [1] (408) 738 4832
Mail: Prime Time Freeware
415-112 N. Mary Ave., Suite 50
Sunnyvale, CA 94086
USA
Many Linux CDroms include a copy of the FTP site tsx-11.mit.edu which
has Linux binaries for Modula-3.
How to call Modula-3 procedures from a C program?
Calling Modula-3 from C is tricky because M3 has a more elaborate
run-time environment. The simplest solution is to make the main
program M3 and then call C via EXTERNAL routines. Calling back into M3
is then relatively straightforward.
Here's an example. It calls the C code to lodge the identity of the M3
procedure to be called back which avoids having to know the actual
name used by the linker.
First a little M3 module to be called from C (M3code), then a C module
called by the M3 main and calling the M3 module (Ccode), and finally
the main program (Main):
(* M3code.i3 *)
INTERFACE M3code;
IMPORT Ctypes;
PROCEDURE put (a: Ctypes.char_star);
END M3code.
(* M3code.m3 *)
UNSAFE MODULE M3code;
IMPORT Ctypes, IO, M3toC;
PROCEDURE put (a: Ctypes.char_star) =
BEGIN
IO.Put (M3toC.StoT (a) & "\n");
END put;
BEGIN
END M3code.
(* Ccode.i3 *)
<*EXTERNAL*> INTERFACE Ccode;
IMPORT Ctypes;
PROCEDURE set (p: PROCEDURE (a: Ctypes.char_star));
PROCEDURE act (a: Ctypes.char_star);
END Ccode.
/* Ccode.c */
typedef void (*PROC)();
static PROC action;
void set (p)
PROC p;
{
action = p; /* register the M3 procedure */
}
void act (a)
char *a;
{
action (a); /* call the M3 procedure */
};
(* Main.m3 *)
UNSAFE MODULE Main;
IMPORT Ccode, M3code, M3toC;
BEGIN
Ccode.set (M3code.put);
Ccode.act (M3toC.TtoS ("Hello world"));
END Main.
(* m3makefile *)
import(libm3)
interface ("Ccode")
c_source ("Ccode")
module ("M3code")
implementation("Main")
program("mixed")
Can Modula-3 code call C++ and vice-versa?
There is no problem to call C++ functions declared as extern C. You
must use a C++ aware linker (e.g. the C++ compiler). A complete
example of M3 calling C++ objects, which in turn call M3 callbacks, is
available in [22]the sgml library.
On some platforms, a call to get the static variables constructors
called may be required:
From: gwyant@cloyd.East.Sun.COM (Geoffrey Wyant - Sun Microsystems
Labs BOS)
You must use your C++ compiler as the linker, rather than /bin/cc or
/bin/ld.
You need to call the function '_main'. The easiest way to do this is
to have the following set of interfaces and implementations:
INTERFACE CXXMain;
<*EXTERN "_main"*> CxxMain;
END CXXMain;
MODULE CXXMain;
BEGIN
CxxMain();
END;
and then import CXXMain into your M3 main module. This will ensure
that the C++ function _main gets called.
How to copy heap objects?
Deep copies are easily performed using Pickles. An object graph is
Pickled to a text writer into a TEXT. Then, a copy is created by
unpickling a new object graph from a text reader created from the
TEXT.
Shallow copies are less often needed but may be performed with the
following procedure:
PROCEDURE Duplicate (r: REFANY): REFANY =
VAR
tc := TYPECODE (r);
n_dims : INTEGER;
res : REFANY;
shape : RTHeapRep.ArrayShape;
BEGIN
IF (r = NIL) THEN RETURN NIL END;
(* allocate a new object of the same type (and shape) as the old one *)
RTHeapRep.UnsafeGetShape (r, n_dims, shape);
IF (n_dims <= 0)
THEN res := RTAllocator.NewTraced (tc);
ELSE res := RTAllocator.NewTracedArray (tc, SUBARRAY(shape^, 0, n_dims));
END;
(* copy the old data into the new object *)
RTMisc.Copy (RTHeap.GetDataAdr (r), RTHeap.GetDataAdr (res),
RTHeap.GetDataSize (r));
RETURN res;
END Duplicate;
How to get output messages to appear immediately (flushing writers)?
Modula-3 Writers are buffered. Thus, you need to issue a Wr.Flush when
the output should appear immediately, for instance to prompt the user
for some input. Since this can become annoying, libraries in other
languages sometimes offer the option of unbuffered writes. In
Modula-3, an equivalent behavior is obtained with AutoFlushWr which
gets a background thread to flush a writer at a specified interval.
How to read a single character as soon as typed?
Characters typed on the keyboard are usually buffered. They become
visible to the reading program only when the buffer is full or after,
for example, a carriage return is received. This is not specific to
Modula-3. To access the characters as they are typed, single character
commands in a full screen editor for example, the input reader must be
configured properly.
From: [23]rrw1000@cus.cam.ac.uk (Richard Watts)
The POSIX way of doing it is to use tcsetattr(), and here is some code
that does it under Solaris 2.x (this was written for serial ports, but
the same thing applies) :
PROCEDURE Open(port : CHAR; timeout : INTEGER := 30) : T RAISES {Error} =
VAR
term : TcPosix.termios;
file : TEXT;
fd : T;
rc : INTEGER;
BEGIN
(* Figure out which device we want to open : *)
CASE port OF
'A' => file := "/dev/ttya";
| 'B' => file := "/dev/ttyb";
ELSE RAISE Error("Invalid port " & Fmt.Char(port) & " specified.\n");
END;
(* Open it. 700 is a good default mode for serial ports. *)
fd := Unix.open(M3toC.TtoS(file), Unix.O_RDWR
, 8_700);
IF fd = -1 THEN
RAISE Error("Open() on " & file & " failed.\n");
END;
(* Get the termios structure for it *)
rc := TcPosix.tcgetattr(fd, ADR(term));
IF rc # 0 THEN
EVAL Unix.close(fd);
RAISE Error("Couldn't get terminal attributes for " & file & ".\n");
END;
(* Modify the termios structure *)
(* The default baud rate is right, but we'd better set it anyway
in case someone left it set up wrong : *)
rc := TcPosix.cfsetospeed(ADR(term), TcPosix.B9600);
IF rc # 0 THEN
EVAL Unix.close(fd);
RAISE Error("Couldn't set output speed for " & file & "\n");
END;
rc := TcPosix.cfsetispeed(ADR(term), TcPosix.B9600);
IF rc # 0 THEN
EVAL Unix.close(fd);
RAISE Error("Couldn't set input speed for " & file & "\n");
END;
(* Modify the line discipline - reset ECHO and ICANON *)
term.c_lflag := Word.And( term.c_lflag,
Word.Not(
Word.Or(TcPosix.ICANON,
TcPosix.ECHO)));
term.c_cc[TcPosix.VMIN] := 0;
term.c_cc[TcPosix.VTIME] := 0; (* Set up timing right *)
(* Now reset the terminal attributes *)
rc := TcPosix.tcsetattr(fd, TcPosix.TCSANOW, ADR(term));
IF rc # 0 THEN
EVAL Unix.close(fd);
RAISE Error("Can't set attributes for " & file & "\n");
END;
RETURN fd;
END Open;
(TcPosix.i3 is one of my interfaces, not libm3's, and I'll supply it
if you like, but it's just a wrapper to tcgetattr and friends. The
baud rate stuff shouldn't be necessary for terminals (or serial
ports..) ). You should be able to somehow get an Rd.T out of this, I
think, but it may involve a bit of hacking. The University of
Cambridge can't have these opinions even if it wants them.
Why is Hello World larger in Modula-3 than in C?
Modula-3 programs are slightly larger than C programs because the
generated code includes runtime type information, and runtime checks
for out-of-bound array references and NIL pointers. Many of these
checks could be removed by a more sophisticated compiler.
The fixed runtime is substantially larger (there is no runtime support
in C). It contains a garbage collector, a thread runtime, and
exception support. It is typically placed in a dynamically linked
library, shared on disk and in memory between all the Modula-3
programs.
What is SRC Modula-3?
[24]SRC-Modula-3 was built by the DEC Systems Research Center and is
freely available and redistributable, with source code. In Europe it
is also available from ftp-i3.informatik.rwth-aachen.de in
pub/Modula-3. The most recent version is release 3.6
The DEC SRC Modula-3 contains the following:
* A native code compiler: uses the GCC backend; on
machines/operating systems that have self-describing stacks, an
optimized exception handling mechanism is provided, on other
architectures, setjmp/longjmp is used. A very fast integrated
backend is available on some platforms (currently NT386 and Linux
i386).
The compilation system provides for minimal recompilation. Only
those units that depend on the modified interface item will be
recompiled.
* m3build: tool that performs dependency analysis and builds the
Modula-3 programs and libraries.
* m3gdb: a Modula-3 aware version of GDB.
* Several tools for performance and coverage analysis.
* A large standard library (libm3) providing
+ A multithread, incremental, generational, conservative
garbage collector
+ Text manipulation.
+ Generic Containers: Lists, Sequences, Tables, SortedLists,
SortedTables
+ Atoms and Symbolic expressions (Lisp like lists)
+ An extensible stream IO system
+ Typesafe binary object transcription (persistent objects)
+ Operating system interfaces
+ Portable interfaces to the language runtime
All standard libraries are thread-friendly. Modula-3 can readily
link with existing C libraries; many libraries including X11R4 and
various UNIX libraries are available as part of libm3.
* Several other libraries for designing graphical user interfaces
and distributed applications.
Why are there strange pragmas for Locking levels and other properties?
The Trestle (ui library) interfaces contain Locking level pragmas. The
base interfaces (libm3 library) contain SPEC pragmas. These are not
processed by the compiler. Instead the Extended Static Checker,
currently under development at DEC SRC, will report on problems
detected based on the program content and the information specified in
these pragmas [25][ESC]. The Extended Static Checker is not yet
available, it may be some time in the future.
Design Issues
Why objects and interfaces?
Allan Heydon on comp.lang.modula3, May 4th 1993:
Modula-3 provides two separate mechanisms for data-hiding: one for
hiding details about how interfaces are implemented, and the other for
hiding details about how objects are implemented.
The first data-hiding mechanism is realized by the distinction between
interfaces and modules. Clients can only import interfaces, so the
names declared in the modules implementing those interfaces are hidden
from clients. Note that this mechanism has only two levels; a name is
either declared in an interface, or it isn't. If a name is only
declared in a module, it can't be used by a client.
The second data-hiding mechanism is realized by opaque types and
revelations. A Modula-3 interface may declare an object type to be
opaque, in which case only a subset of the fields and methods of that
object are revealed to clients importing the interface. Furthermore,
the Modula-3 revelation mechanism allows a designer to reveal
successively more fields and methods of an object in a series of
interfaces. The fields and methods visible to a client then depends on
which interfaces the client imports.
The latter mechanism is quite flexible. As opposed to the
interface/module data-hiding mechanism, opaque types allow you to
define an arbitrary number of levels at which more and more
information about the implementation of your object is revealed.
See Sections 2.2.10, 2.4.6, and 2.4.7 of "Systems Programming with
Modula-3" for more information about opaque types and about partial
and complete revelations.
What is the purpose of BRANDED and REVEAL?
Allan Heydon writes:
These two keywords are necessary because of two quite different
features of the language. REVEAL is necessary because Modula-3 has
opaque types and partial revelations. BRANDED is necessary because the
Modula-3 type system uses structural equivalence instead of name
equivalence.
In Modula-3, the concrete structure of a type can be hidden from
clients in an interface. A common idiom is:
INTERFACE I;
TYPE
T <: TPublic;
TPublic = OBJECT
(* fields *)
METHODS
(* methods *)
END;
END I.
The line "T <: TPublic" introduces the type "I.T" as an opaque subtype
of the type "I.TPublic". It does not reveal any of the other details
of the concrete structure of "I.T" to clients. Hence, "I.T" is said to
be an opaque type. Put another way, the structure of "I.T" is only
partially revealed to clients.
In addition, it is possible to reveal more of "I.T"'s structure in
other interfaces, like this:
INTERFACE IRep;
IMPORT I;
TYPE
TPrivate = I.TPublic OBJECT
(* more fields *)
METHODS
(* more methods *)
END;
REVEAL
I.T <: TPrivate;
END IRep.
This interface declares a type "IRep.TPrivate" that is a subtype of
"I.TPublic". It also asserts that "I.T" is also a subtype of
"IRep.TPrivate". A client that imports only the interface "I" has
access only to the fields and methods in "I.TPublic" when accessing an
object of type "I.T", but a client that imports both "I" and "IRep"
also has access to the fields and methods in "IRep.TPrivate" when
accessing an object of type "I.T".
The "REVEAL" statement in this module simply asserts a subtype
relation. Unlike type declarations, revelations introduce no new
names. Hence, we could not have used the "TYPE" keyword in this case
because the type "I.T" has already been declared once (albeit
opaquely) in interface "I".
Every opaque type must have a complete revelation. A complete
revelation has the form:
REVEAL
T = TConcrete;
The revelation specifies that "TConcrete" is the concrete type for the
opaque type "T".
The Modula-3 type system uses structural equivalence instead of name
equivalence. This means that two types are equal iff they have the
same structure. One consequence of this rule is that two types you
might intend to be distinct may actually be equal. This can have
unintended effects on the run-time behavior of your program. For
example, if both types that you expect to be distinct are actually
structurally equivalent and the two types guard two arms of a TYPECASE
statement, the arm for the second type will never be taken.
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