Modula-2

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Modula-2
Paradigm: imperative, structured, modular
Appeared in: 1978
Designed by: Niklaus Wirth
Typing discipline: strong, static
Major implementations: Gardens Point, p1, Native XDS-x86
Dialects: PIM2, PIM3, PIM4, ISO
Influenced by: Pascal, Mesa, ALGOL
Influenced: Modula-3, Oberon, Objective Modula-2, Ada, Zonnon

Modula-2 is a computer programming language invented by Niklaus Wirth at ETH around 1978, as a successor to Modula, an intermediate language by him. Modula-2 was implemented in 1980 for the Lilith computer, commercialized in 1982 by startup company DISER (Data-Image-Sound-Processor-and Emitter-Receiver-System) as MC1 and MC2. DISER sold 120 units worldwide. The Modula-2 language is loosely based upon the Mesa programming language, from Xerox, that Wirth saw during his 1976 sabbatical year at Xerox PARC.

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[edit] Description

Modula-2 is a general purpose procedural language, sufficiently flexible to do systems programming, but with much broader application. In particular, it was designed to support separate compilation and data abstraction in a straightforward way. Much of the syntax is based on Wirth's earlier and better-known language, Pascal. Modula-2 was designed to be broadly similar to Pascal, with some elements removed and the important addition of the module concept, and direct language support for multiprogramming.

The Modula-2 module may be used to encapsulate a set of related subprograms and data structures, and restrict their visibility from other portions of the program. The module design implemented the data abstraction feature of Modula-2 in a very clean way. Modula-2 programs are composed of modules, each of which is made up of two parts: a definition module, the interface portion, which contains only those parts of the subsystem that are exported (visible to other modules), and an implementation module, which contains the working code that is internal to the module.

The language has strict scope control. In particular the scope of a module can be considered as an impenetrable wall: Except for standard identifiers no object from the outer world is visible inside a module unless explicitly imported; no internal module object is visible from the outside unless explicitly exported.

Suppose module M1 exports objects a, b, c, and P by enumerating its identifiers in an explicit export list

 DEFINITION MODULE M1;
   EXPORT QUALIFIED a, b, c, P;
   ...

Then the objects a, b,c, and P from module M1 become now known outside module M1 as M1.a, M1.b, M1.c, and M1.P. They are exported in a qualified manner to the universe (assumed module M1 is global). The exporting module's name, i.e. M1, is used as a qualifier followed by the object's name.

Suppose module M2 contains the following IMPORT declaration

 MODULE M2;
   IMPORT M1;
   ...

Then this means that the objects exported by module M1 to the universe can now be used inside module M2. They are referenced in a qualified manner like this: M1.a, M1.b, M1.c, and M1.P. Example:

   ...
   M1.a := 0; M1.c := M1.P (M1.a + M1.b);
   ...

Qualified export avoids name clashes: For instance, if another module M3 would also export an object called P, then we can still distinguish the two objects, since M1.P differs from M3.P. Thanks to the qualified export it does not matter that both objects are called P inside their exporting modules M1 and M3.

There is an alternative technique available, which is in wide use by Modula-2 programmers. Suppose module M4 is formulated as this

 MODULE M4;
   FROM M1 IMPORT a, b, c, P;

Then this means that objects exported by module M1 to the universe can again be used inside module M4, but now by mere references to the exported identifiers in an "unqualified" manner like this: a, b, c, and P. Example:

   ...
   a := 0; c := P (a + b);
   ...

This technique of unqualifying import allows to use objects outside their exporting module in exactly the same simple, i.e. unqualified, manner as inside the exporting module. The walls surrounding all modules have now become irrelevant for all those objects for which this has been explicitly allowed. Of course unqualifying import is only applicable if there are no name clashes.

These export and import rules may seem unnecessarily restrictive and verbose. But they do not only safeguard objects against unwanted access, but also have the pleasant side-effect of providing automatic cross-referencing of the definition of every identifier in a program: if the identifier is qualified by a module name, then the definition comes from that module. Otherwise if it occurs unqualified, simply search backwards, and you will either encounter a declaration of that identifier, or its occurrence in an IMPORT statement which names the module it comes from. This property becomes very useful when trying to understand large programs containing many modules.

The language provides for (limited) single-processor concurrency (monitors, coroutines and explicit transfer of control) and for hardware access (absolute addresses, bit manipulation, and interrupts). It uses name equivalence.

[edit] Dialects

There are two major dialects of Modula-2. The first is PIM, named after the book "Programming in Modula-2" by Niklaus Wirth. There were three major editions of PIM, the second, third (corrected) and fourth editions, each describing slight variants of the language. The second major dialect is ISO, from the standardization effort by the International Organization for Standardization.

  • PIM2 (1983)
    • Required explicit EXPORT clause in definition modules.
  • PIM3 (1985)
    • Removed the EXPORT clause from definition modules by the observation that everything within a definition module defines the interface to that module, hence the EXPORT clause was redundant.
  • PIM4
    • The behaviour of the MOD and REM operators were specified when their operands were negative.
  • ISO
    • ISO Modula-2 resolved most of the ambiguities in PIM Modula-2. It added the data types COMPLEX and LONGCOMPLEX, exceptions, module termination (FINALLY clause) and a complete standard I/O library. There are numerous minor differences and clarifications.

[edit] Related languages

Although Modula-2 is by far the best-known and most widely used variant, there are several languages which are related in one way or another: the original, and quite different, Modula (intended for systems implementation), Modula-2+, Modula-2* (parallel extension), ISO Modula-2's OO and generic extensions, Modula-3 (by DEC and Olivetti; adding garbage collection, objects, and generics), Oberon (another, later, Wirth design), Oberon-2 (Oberon with OO extensions), Objective Modula-2 (a hybrid of Modula-2 and Smalltalk) and a number of others. These should not be regarded as "better versions" or "replacements" for Modula-2; most are different languages with different purposes, and with strengths and weaknesses of their own.

Modula-2 was developed as the system language for the Lilith workstation, and formed the ancestor for the Oberon language and workstation project (System Oberon) developed at ETH Zürich. Many current programming languages have adopted features of Modula-2.

[edit] Current compilers

[edit] Discontinued compilers

[edit] Books

[edit] External links

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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