Modula-3

Paradigms	imperative, structured, procedural, modular, concurrent, object-oriented, generic
Family	Wirth/Modula
Designed by	Luca Cardelli, James Donahue, Lucille Glassman, Mick Jordan; Bill Kalsow, Greg Nelson
Developers	DEC Olivetti elego Software Solutions GmbH
First appeared	1988 (1988)
Stable release	5.8.6 / July 14, 2010 (2010-07-14)
Preview release	5.8.6 / July 14, 2010 (2010-07-14)
Typing discipline	strong, static, safe or if unsafe explicitly safe isolated
Scope	Lexical
Platform	IA-32, x86-64, PowerPC, SPARC
OS	Cross-platform: FreeBSD, Linux, Darwin, SunOS
Website	www.modula3.org
Major implementations
SRC Modula-3, CM3,[1] PM3,[2] EZM3,[3] M3/PC Klagenfurt[4]
Influenced by
ALGOL, Euclid, Mesa, Modula-2, Modula-2+, Oberon, Pascal
Influenced
C#, Java, Nim,[5] OCaml, Rust,[6] Python[7]

Modula-3 is a programming language conceived as a successor to an upgraded version of Modula-2 known as Modula-2+. While it has been influential in research circles (influencing the designs of languages such as Java, C#, Python^[8] and Nim) it has not been adopted widely in industry. It was designed by Luca Cardelli, James Donahue, Lucille Glassman, Mick Jordan (before at the Olivetti Software Technology Laboratory), Bill Kalsow and Greg Nelson at the Digital Equipment Corporation (DEC) Systems Research Center (SRC) and the Olivetti Research Center (ORC) in the late 1980s.

Modula-3's main features are modularity, simplicity and safety while preserving the power of a systems-programming language. Modula-3 aimed to continue the Pascal tradition of type safety, while introducing new constructs for practical real-world programming. In particular Modula-3 added support for generic programming (similar to templates), multithreading, exception handling, garbage collection, object-oriented programming, partial revelation, and explicit marking of unsafe code. The design goal of Modula-3 was a language that implements the most important features of modern imperative programming languages in quite basic forms. Thus allegedly dangerous and complicating features such as multiple inheritance and operator overloading were omitted.

Historical development

The Modula-3 project started in November 1986 when Maurice Wilkes wrote to Niklaus Wirth with some ideas for a new version of Modula. Wilkes had been working at DEC just prior to this point, and had returned to England and joined Olivetti's Research Strategy Board. Wirth had already moved on to Oberon, but had no problems with Wilkes's team continuing development under the Modula name. The language definition was completed in August 1988, and an updated version in January 1989. Compilers from DEC and Olivetti soon followed, and 3rd party implementations after that.

Its design was heavily influenced by work on the Modula-2+ language in use at SRC and at the Acorn Computers Research Center (ARC, later ORC when Olivetti acquired Acorn) at the time, which was the language in which the operating system for the DEC Firefly multiprocessor VAX workstation was written and in which the Acorn Compiler for Acorn C and Modula Execution Library (CAMEL) at ARC for the ARX operating system project of ARM based Acorn Archimedes range of computers was written. As the revised Modula-3 Report states, the language was influenced by other languages such as Mesa, Cedar, Object Pascal, Oberon and Euclid.^[9]

During the 1990s, Modula-3 gained considerable currency as a teaching language, but it was never widely adopted for industrial use. Contributing to this may have been the demise of DEC, a key Modula-3 supporter (especially when it ceased to maintain it effectively before DEC was sold to Compaq in 1998). In any case, in spite of Modula-3's simplicity and power, it appears that there was little demand for a procedural compiled language with restricted implementation of object-oriented programming. For a time, a commercial compiler named CM3 maintained by one of the chief implementors prior at DEC SRC who was hired before DEC being sold to Compaq, an integrated development environment (IDE) named Reactor and an extensible Java virtual machine (licensed in binary code and source code formats and buildable with Reactor) were offered by Critical Mass, Inc., but that company ceased active operations in 2000 and gave some of the source code of its products to elego Software Solutions GmbH. Modula-3 is now taught in universities mostly in comparative programming language courses, and its textbooks are out of print. Essentially the only corporate supporter of Modula-3 is elego, which inherited the sources from Critical Mass and has since made several releases of the CM3 system in source and binary code. The Reactor IDE has been open source released after several years it had not, with the new name CM3-IDE. In March 2002, elego also took over the repository of another active Modula-3 distribution, PM3, until then maintained at the École Polytechnique de Montréal but which later continued by the work on HM3 improved over the years later until it was obsoleted.

Syntax

A common example of a language's syntax is the "Hello, World!" program.

MODULE Main; 
IMPORT IO;
BEGIN
  IO.Put("Hello World\n")
END Main.

All programs in Modula-3 have at least a module file, while most also include an interface file that is used by clients to access data from the module. As in some other languages, a Modula-3 program must export a Main module, which can either be a file named Main.m3, or a file can call EXPORT to export the Main module.

MODULE Foo EXPORTS Main

Module file names are advised to be the same as the name in source code. If they differ, the compiler only emits a warning.

Other conventions in the syntax include naming the exported type of an interface T, since types are usually qualified by their full names, so a type T inside a module named Foo will be named Foo.T. This aids in readability. Another similar convention is naming a public object Public as in the OOP examples below.

Language features

MODULE HelloWorld EXPORTS Main; 
IMPORT IO;
BEGIN
  IO.Put("Hello World\n")
END HelloWorld.

MODULE HelloWorld EXPORTS Main; 
FROM IO IMPORT Put;
BEGIN
  Put("Hello World\n")
END HelloWorld.

INTERFACE IntegerElem;
CONST Name = "Integer";
TYPE T = INTEGER;
PROCEDURE Format(x: T): TEXT;
PROCEDURE Scan(txt: TEXT; VAR x: T): BOOLEAN;
END IntegerElem.

GENERIC INTERFACE GenericStack(Element);
(* Here Element.T is the type to be stored in the generic stack. *)
TYPE
   T = Public OBJECT;
   Public = OBJECT
   METHODS
       init(): TStack;
       format(): TEXT;
       isEmpty(): BOOLEAN;
       count(): INTEGER;
       push(elm: Element.T);
       pop(VAR elem: Element.T): BOOLEAN;
   END;
END GenericStack.

GENERIC MODULE GenericStack(Element);
< ... generic implementation details... >
PROCEDURE Format(self: T): TEXT =
VAR
   str: TEXT;
BEGIN
   str := Element.Name & "Stack{";
   FOR k := 0 TO self.n -1 DO
       IF k > 0 THEN str := str & ", "; END;
       str := str & Element.Format(self.arr[k]);
   END;
   str := str & "};";
   RETURN str;
END Format;
< ... more generic implementation details... >
END GenericStack.

INTERFACE IntegerStack = GenericStack(IntegerElem) END IntegerStack.

MODULE IntegerStack = GenericStack(IntegerElem) END IntegerStack.

INTERFACE A;

TYPE Color = {Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, White};

END A;

MODULE B;

IMPORT A;
FROM A IMPORT Color;

VAR
  aColor: A.Color;  (* Uses the module name as a prefix *)
  theColor: Color;  (* Does not have the module name as a prefix *)
  anotherColor: A.Color;

BEGIN
  aColor := A.Color.Brown;
  theColor := Color.Red;
  anotherColor := Color.Orange;  (* Can't simply use Orange *)
END B.

INTERFACE Person;

TYPE T <: Public;
  Public = OBJECT 
  METHODS
    getAge(): INTEGER;
    init(name: TEXT; age: INTEGER): T;
  END;

END Person.

VAR jim := NEW(Person.T).init("Jim", 25);

MODULE Person;

REVEAL T = Public BRANDED 
OBJECT 
  name: TEXT;   (* These two variables *)
  age: INTEGER; (* are private. *)
OVERRIDES
  getAge := Age;
  init := Init;
END;

PROCEDURE Age(self: T): INTEGER =
  BEGIN
    RETURN self.age;
  END Age;

PROCEDURE Init(self: T; name: TEXT; age: INTEGER): T =
  BEGIN
    self.name := name;
    self.age := age;
  RETURN self;
  END Init;

BEGIN
END Person.

Standard library features

Continuing a trend started with the C language, many of the features needed to write real programs were left out of the language definition and instead provided via a standard library set. Most of the interfaces below are described in detail in^[11]

Standard libraries providing the following features. These are called standard interfaces and are required (must be provided) in the language.

• Text: Operations on immutable string references, called TEXTs
• Thread: Operations relating to threading, including MUTEX, condition variable, and thread pausing. The threading library provides pre-emptive threads switching
• Word: Bitwise operations on unsigned integers (or machine words). Normally implemented directly by the compiler
• Floating-point interfaces

Some recommended interfaces implemented in the available implementations but are not required

• Lex: For parsing number and other data
• Fmt: Formatting various datatypes for printing
• Pkl (or Pickle): Object serialization of any reference types reachable by the garbage collector
• Table: Generic modules for maps

As in C, I/O is also provided via libraries, in Modula-3 called Rd and Wr. The object-oriented design of the Rd (readers) and Wr (writers) libraries is covered in detail in the book by Greg Nelson. An interesting aspect of Modula-3 is that it is one of few programming languages which standard libraries have been formally verified not to contain various types of bugs, including locking bugs. This was done under the auspices of the Larch/Modula-3 (see Larch family)^[12] and Extended static checking^[13] projects at DEC Systems Research Center.

Implementations

Several compilers are available, most of them open source.

• DEC-SRC M3, the original.^[14]
• Olivetti Research Center (ORC) Modula-3 toolkit, originally a compiler, now available as a library for syntactic, lexical and semantic analysis of Modula-3 programs.^[15]
• Critical Mass CM3, a different successor of DEC-SRC M3
• Polytechnique Montreal Modula-3 PM3, a successor of DEC-SRC M3, currently merging with CM3
• EzM3, an independent lightweight and easily portable implementation, developed in connection with CVSup
• HM3, a successor of the pm3-1.1.15 release of PM3, with support of native threading using NPTL
• CM3, the successor to Critical Mass CM3. This is the only up to date, maintained and developed implementation. Releases are available from http://www.opencm3.net/releng/.

Since the only aspect of C data structures that is missing from Modula-3 is the union type, all extant Modula-3 implementations are able to provide good binary code compatibility with C language type declarations of arrays and structs.

Books

None of these books are still in print, although used copies are obtainable and some are digitized, partly or fully, and some chapters of one them have prior or posterior versions obtainable as research reports from the web.

• Greg Nelson, ed., Systems Programming with Modula-3 The definitive reference on the Modula-3 language with interesting articles on object-oriented systems software construction and a documentation of the discussion leading to the final features of the language. There are some formerly (see^[9] for Chapter two,^[16] for chapter four,^[17] for chapter five,^[18] for chapter six) and some posteriorly (see^[19] for Chapter one and more updated two, thus of both prior versions of language definition^[9] and,^[11] for chapter three and^[20] for chapter seven) of publishing versions of the majority of its eight chapters individually available from prior DEC Systems Research Center (SRC) as research reports for download.
• Samuel P. Harbison, Modula-3 Easy to use class textbook.
• Robert Sedgewick, Algorithms in Modula-3
• Laszlo Boszormenyi & Carsten Weich, Programming in Modula-3: An Introduction in Programming with Style
• Renzo Orsini, Agostino Cortesi Programmare in Modula-3: introduzione alla programmazione imperativa e a oggetti an Italian book of the language explaining its main features.

Projects using Modula-3

Software which is programmed Modula-3 includes:

• The SPIN operating system
• The CVSup software repository synchronizing program
• The Obliq language, which uses Modula-3 network objects ability to migrate objects over local networks transparently, allowing a distributed ability to Modula-3 object-oriented programming paradigm. It has been used to build distributed applications, computer animations, and web programming applications in the form of scripting extension to Modula-3.

Influences on other programming languages

Although Modula-3 did not gain mainstream status, several parts of the DEC-SRC M3 distribution did. Probably the most influential part was the Network Objects library, which formed the basis for Java's first Remote Method Invocation (RMI) implementation, including the network protocol. Only when Sun moved from the Common Object Request Broker Architecture (CORBA) standard to the IIOP based protocol was it dropped. The Java documentation on garbage collection of remote objects still refer to the pioneering work done for Modula-3 Network Objects.^[21] Python's implementation of classes was also inspired by the class mechanism found in C++ and Modula-3.^[22] Also the language Nim makes use of some aspects of Modula-3, such as traced vs untraced pointers.

References

External links

• Official website
• Modula3 on GitHub
• CM3 Implementation Website
• Modula-3 Home Page (now long dead, mirror)
• Modula-3: Language definition
• elego Software Solutions
• Modula-3 newsgroup, mostly deserted
• Modula-3 Development Mailing List, active
• Notes from Caltech's CS2 class, taught in Modula-3 in 2002 and 2003
• Caltech's CS3 class 2009 at the Wayback Machine (archived May 23, 2013)
• mirror Programming in Modula-3: program examples
• Building Distributed OO Applications: Modula-3 Objects at Work. Michel R. Dagenais. Draft Version (January 1997)
• Modula-3: Language, Libraries and Tools. Presentation on Modula-3 over 120 slides. Michael R. Dagenais, dead
• Object-Oriented Data Abstraction in Modula-3. Joseph Bergin (1997)
• Computerworld Interview with Luca Cardelli on Modula-3