| Appeared in | 1970s and 80s |
|---|---|
| Developer | Xerox PARC |
| Typing discipline | strongly typed |
| Influenced by | ALGOL |
| Influenced | Java, Modula-2, Cedar |
Mesa was an innovative programming language developed in the late 1970s at the Xerox Palo Alto Research Center in Palo Alto, California, United States.[1] The language was named after the mesas of the American Southwest, referring to its design intent to be a "high-level" programming language.
Mesa is an ALGOL-like language with strong support for modular programming. Every library module has at least two source files: a definitions file specifying the library's interface plus one or more program files specifying the implementation of the procedures in the interface.[2] To use a library, a program or higher-level library must "import" the definitions. The Mesa compiler type-checks all uses of imported entities; this combination of separate compilation with type-checking was unusual at the time.
Mesa introduced several other innovations in language design and implementation, notably in the handling of software exceptions, thread synchronization, incremental compilation, and more.
Mesa was developed on the Xerox Alto, one of the first personal computers with a graphical user interface, however most of the Alto's system software was written in BCPL. Mesa was the system programming language of the later Xerox Star workstations, and for the GlobalView desktop environment. Xerox PARC developed Cedar, which was based on Mesa, with a number of additions including garbage collection, better string support, called Ropes, and a native compiler for Sun SPARC workstations.
Mesa had a major influence on the design of other important languages, such as Modula-2 and Java, and was an important vehicle for the development and dissemination of the fundamentals of GUIs, networked environments, and the other advances Xerox contributed to the field of computer science.
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Mesa was originally designed in the Computer Systems Laboratory (CSL), a branch of the Xerox Palo Alto Research Center, for the Alto, an experimental micro-coded workstation. Initially its spread was confined to PARC and a few universities to which Xerox had donated some Altos.
Mesa was later adopted as the system's programming language for Xerox's commercial workstations such as the Xerox 8010 (Xerox Star, Dandelion) and Xerox 6085 (Daybreak), in particular for the Pilot operating system.
A secondary development environment, called the Xerox Development Environment (XDE) allowed developers to debug both the operating system Pilot as well as ViewPoint GUI applications using a world swap mechanism. This allowed the entire "state" of the world to be swapped out, and allowed low level system crashes which paralyzed the whole system to be debugged. The Pilot/Mesa world in later releases moved away from the world swap view when the micro-coded machines were phased out in favor of SPARC workstations and Intel PC's running a Mesa PrincOps emulator for the basic hardware instruction set.
Mesa was taught via the Mesa Programming Course that took people through the wide range of technology Xerox had available at the time and ended with the programmer writing a "hack", a workable program designed to be useful. An actual example of such a hack is the BWSMagnifier, which was written in 1988 and allowed people to magnify sections of the workstation screen as defined by a resizable window and a changeable magnification factor. Trained Mesa programmers from Xerox were well versed in the fundamental of GUIs, networking, exceptions, and multi-threaded programming, almost a decade before they became standard tools of the trade.
Within Xerox, Mesa was eventually superseded by the Cedar programming language. Many Mesa programmers and developers left Xerox in 1985; some of them went to DEC Systems Research Center where they used their experience with Mesa in the design of Modula-2+, and later of Modula-3.
Mesa was a strongly typed programming language with type-checking across module boundaries, but with enough flexibility in its type system that heap allocators could be written in Mesa.[3]
Because of its strict separation between interface and implementation, Mesa allows true incremental compilation and encourages architecture- and platform-independent programming. They also simplified source-level debugging, including remote debugging via the Ethernet.
Mesa had rich exception-handling facilities, with four types of exceptions. It has support for thread synchronization via monitors. Mesa was the first language to implement monitor BROADCAST, a concept introduced by the Pilot operating system.[4]
Mesa has an "imperative" and "algebraic" syntax, in many respects more similar to ALGOL and Pascal than to C. For instance, compound commands are indicated by BEGIN/END keywords, rather than braces. In Mesa, all keywords are written in uppercase.[1]
Due to a peculiarity of the Alto's keyboard, Mesa's original character set did not include the underscore, so programmers were forced to use CamelCase for compound identifiers --- a practice which was incorporated in PARC's standard programming style. On the other hand, Mesa designers adopted the left-pointing arrow '←' character, which took the place of underscore in the Alto keyboard, for the assignment operator.
When the Mesa designers wanted to implement an exception facility, they hired a recent M.S. graduate from Colorado who had written his thesis on exception handling facilities in algorithmic languages. This led to the richest exception facility for its time, with primitives SIGNAL, ERROR, ABORT, RETRY, CATCH, and CONTINUE. Because the language did not have type-safe checks to verify proper signal handling, uncaught exceptions were a common cause of bugs in released software.