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Forth is often spoken of as a language because that is its most visible aspect. But in fact, Forth is both more and less than a conventional programming language: more in that all the capabilities normally associated with a large portfolio of separate programs (compilers, editors, etc.) are included within its range and less in that it lacks (deliberately) the complex syntax characteristic of most high-level languages.
The original implementations of Forth were stand-alone systems that included functions normally performed by separate operating systems, editors, compilers, assemblers, debuggers and other utilities. A single simple, consistent set of rules governed this entire range of capabilities. Today, although very fast stand-alone versions are still marketed for many processors, there are also many versions that run co-resident with conventional operating systems such as MS-DOS and UNIX.
Forth is not derived from any other language. As a result, its appearance and internal characteristics may seem unfamiliar to new users. But Forth's simplicity, extreme modularity, and interactive nature offset the initial strangeness, making it easy to learn and use. A new Forth programmer must invest some time mastering its large command repertoire. After a month or so of full-time use of Forth, that programmer could understand more of its internal working than is possible with conventional operating systems and compilers.
The most unconventional feature of Forth is its extensibility. The programming process in Forth consists of defining new words - actually new commands in the language. These may be defined in terms of previously defined words, much as one teaches a child concepts by explaining them in terms of previously understood concepts. Such words are called high-level definitions. Alternatively, new words may also be defined in assembly code, since most Forth implementations include an assembler for the host processor.
This extensibility facilitates the development of special application languages for particular problem areas or disciplines.
Forth's extensibility goes beyond just adding new commands to the language. With equivalent ease, one can also add new kinds of words. That is, one may create a word which itself will define words. In creating such a defining word the programmer may specify a specialized behavior for the words it will create which will be effective at compile time, at run-time, or both. This capability allows one to define specialized data types, with complete control over both structure and behavior. Since the run-time behavior of such words may be defined either in high-level or in code, the words created by this new defining word are equivalent to all other kinds of Forth words in performance. Moreover, it is even easy to add new compiler directives to implement special kinds of loops or other control structures.
Most professional implementations of Forth are written in Forth. Many Forth systems include a meta-compiler which allows the user to modify the internal structure of the Forth system itself.
The system was so useful that astronomers from all over the world began asking for copies. Its use spread rapidly, and in 1976 Forth was adopted as a standard language by the International Astronomical Union.
In 1973, Moore and colleagues formed FORTH, Inc. to explore commercial uses of the language. FORTH, Inc. developed multi-user versions of Forth on minicomputers for diverse projects ranging from data bases to scientific applications such as image processing. In 1977, FORTH, Inc. developed a version for the newly introduced 8-bit microprocessors called microFORTH, which was successfully used in embedded microprocessor applications in the United States, Britain and Japan.
Stimulated by the volume marketing of microFORTH, a group of computer hobbyists in Northern California became interested in Forth, and in 1978 formed the Forth Interest Group (FIG). They developed a simplified model which they implemented on several microprocessors and published listings and disks at very low cost. Interest in Forth spread rapidly, and today there are chapters of the Forth Interest Group throughout the U.S. and in over fifteen countries.
By 1980, a number of new Forth vendors had entered the market with versions of Forth based upon the FIG model. Primarily designed for personal computers, these relatively inexpensive Forth systems have been distributed very widely.
In the early 1980's, Rockwell produced a 6502-variant with Forth primitives in on-board ROM, the Rockwell 65F11. This chip has been used successfully in many embedded microprocessor applications. In the mid-1980's Zilog developed the z8800 (Super8) which offered ENTER (nest), EXIT (unnest) and NEXT in microcode.
In 1981, Moore undert t performs the text interpretation, and if CATCH returns an exception code, the file may be closed and the exception reTHROWn so that the files being included at an outer nesting level may be closed also. Note that the Standard allows, but does not require, INCLUDE-FILE to close its open files if an exception occurs. However, it does require INCLUDE-FILE to unnest the input source specification if an exception is THROWn.
If THROW is executed with a non zero argument, the effect is as if the corresponding CATCH had returned it. In that case, the stack depth is the same as it was just before CATCH began execution. The values of the i*x stack arguments could have been modified arbitrarily during the execution of xt. In general, nothing useful may be done with those stack items, but since their number is known (because the stack depth is deterministic), the application may DROP them to return to a predictable stack state.
Typical use:
: could-fail ( -- char )
KEY DUP [CHAR] Q = IF 1 THROW THEN ;
: do-it ( a b -- c) 2DROP could-fail ;
: try-it ( --)
1 2 ['] do-it CATCH IF ( x1 x2 )
2DROP ." There was an exception" CR
ELSE ." The character was " EMIT CR
THEN
;
: retry-it ( -- )
BEGIN 1 2 ['] do-it CATCH WHILE
( x1 x2) 2DROP ." Exception, keep trying" CR
REPEAT ( char )
." The character was " EMIT CR
;
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