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Modula-2

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Modula-2 is a computer programming language invented by Niklaus Wirth at ETH around 1978, as a successor to Modula, another language by him that was never implemented. 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.

Description

Modula-2 is a general purpose procedural language, sufficiently flexible to do systems programing, 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 inpenetrable wall: Except for standard idenfiers 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.

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), 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.

Along with C and Ada, Modula-2 is often regarded as one of the three most significant "modern" programming languages (ie, of the structured programming era). Of the three, Modula-2 is the smallest, easiest to read, easiest to learn, and cleanest designed, both syntactically and conceptually. Modula-2 was developed as the system language for the Lilith workstation, and formed the basis of the Oberon project at ETHZ. Most current programming languages have adopted many of the features of Modula-2.

Current compilers

Discontinued compilers

  • M2Amiga for Amiga
  • Logitech had a series of Modula-2 compilers
  • Stony Brook Modula-2
  • TopSpeed (aka Clarion, aka JPI, aka Jensen and Partners) had several good 16-bit Modula-2 compilers, with nice productive IDE's. However to my knowledge they are not available anymore. For a while they came with the Clarion product though. This was the "Turbo Pascal" of Modula-2. Slightly non standard, but very popular both in business and education, an excellent toolchain.

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

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