Modula 3: История и особенности языка программирования

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 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.

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. 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 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 , 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, 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.

Some ability is deemed unsafe, where the compiler can no longer guarantee that results will be consistent; for example, when interfacing to the C language. The keyword UNSAFE prefixed in front of INTERFACE or MODULE, may be used to tell the compiler to enable certain low level features of the language. For example, an unsafe operation is bypassing the type system using LOOPHOLE to copy the bits of an integer into a floating point REAL number. An interface that imports an unsafe module must also be unsafe. A safe interface may be exported by an unsafe implementation module. This is the typical use when interfacing to external libraries, where two interfaces are built: one unsafe, the other safe.

Modula 3 supports the allocation of data at runtime. There are two kinds of memory that can be allocated, TRACED and UNTRACED, the difference being whether the garbage collector can see it or not. NEW is used to allocate data of either of these classes of memory. In an UNSAFE module, DISPOSE is available to free untraced memory.

Exception handling is based on a TRY EXCEPT block system, which has since become common. One feature that has not been adopted in other languages, with the notable exceptions of Delphi, Python, Scala and Visual Basic. NET, is that the EXCEPT construct defined a form of switch statement with each possible exception as a case in its own EXCEPT clause. Modula 3 also supports a LOOP EXIT END construct that loops until an EXIT occurs, a structure equivalent to a simple loop inside a TRY EXCEPT clause.

The language supports the use of multi threading, and synchronization between threads. There is a standard module within the runtime library (m3core) named Thread, which supports the use of multi threaded applications. The Modula 3 runtime may make use of a separate thread for internal tasks such as garbage collection. A built in data structure MUTEX is used to synchronize multiple threads and protect data structures from simultaneous access with possible corruption or race conditions. The LOCK statement introduces a block in which the mutex is locked. Unlocking a MUTEX is implicit by the code execution locus's leaving the block. The MUTEX is an object, and as such, other objects may be derived from it. For example, in the input/output (I/O) section of the library libm3, readers and writers (Rd. T, and Wr. T) are derived from MUTEX, and they lock themselves before accessing or modifying any internal data such as buffers.

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. Python's implementation of classes was also inspired by the class mechanism found in C++ and Modula 3. Also the language Nim makes use of some aspects of Modula 3, such as traced vs untraced pointers.