Modula-2 is a computer programming language invented by Niklaus Wirth at ETH, around 1978, as a successor to his intermediate language Modula. Modula-2 was implemented in 1980 for the Lilith computer, which was 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 was understood by Niklaus Wirth as a successor to his first major programming language Pascal. The language design was also influenced by the Mesa programming language and the new programming possibilities of the early personal computer Xerox Alto, both from Xerox, that Wirth saw during his 1976 sabbatical year at Xerox PARC.
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
Then this means that the objects exported by module M1 to the universe of its enclosing program 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
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 use of variables and other 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 usable 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.
PIM3 contained the following 40 reserved words:
AND ELSIF LOOP REPEAT ARRAY END MOD RETURN BEGIN EXIT MODULE SET BY EXPORT NOT THEN CASE FOR OF TO CONST FROM OR TYPE DEFINITION IF POINTER UNTIL DIV IMPLEMENTATION PROCEDURE VAR DO IMPORT QUALIFIED WHILE ELSE IN RECORD WITH
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.
Modula-2 was selected as the basis for Delco's high level language because of its many strengths over other alternative language choices in 1986. After Delco Electronics was spun off from GM (with other component divisions) to form Delphi in 1997, global sourcing required that a non proprietary high level software language be used. ECU embedded software now developed at Delphi use commercial C compilers.