Editing Extensible programming (section)

== Historical movement ==
The first paper usually<ref name="Standish1975">Standish, Thomas A., "[https://pdfs.semanticscholar.org/7f11/082b409647e8d50dadd3a369a10278b5890f.pdf Extensibility in Programming Language Design]", ''SIGPLAN Notices'' 10 no. 7 (July 1975), pp. 18–21.</ref><ref name="Sammet1969">Sammet, Jean E., ''Programming Languages: History and Fundamentals'', Prentice-Hall, 1969, section III.7.2</ref> associated with the extensible programming language movement is M. [[Douglas McIlroy]]'s 1960 paper on [[Macro (computer science)|macros]] for [[high-level programming language]]s.<ref name="McIlroy1960">McIlroy, M.D., "[https://dl.acm.org/citation.cfm?id=367223 Macro Instruction Extensions of Compiler Languages]", ''Communications of the ACM'' 3 no. 4 (April 1960), pp. 214–220.</ref> Another early description of the principle of extensibility occurs in Brooker and Morris's 1960 paper on the [[compiler-compiler]].<ref name="Brooker&Morris1962">Brooker, R.A. and Morris, D., "[https://dl.acm.org/citation.cfm?id=321106 A General Translation Program for Phrase Structure Languages]", ''Journal of the ACM'' 9 no. 1 (January 1962), pp. 1–10. The paper was received in 1960.</ref> The peak of the movement was marked by two academic symposia, in 1969 and 1971.<ref name="Christensen&Shaw1969">Christensen, C. and Shaw, C.J., eds., Proceedings of the Extensible Languages Symposium, ''SIGPLAN Notices'' 4 no. 8 (August 1969).</ref><ref name="Schuman1971">Schuman, S.A., ed., Proceedings of the International Symposium on Extensible Languages, ''SIGPLAN Notices'' 6 no. 12 (December 1971).</ref> By 1975, a survey article on the movement by Thomas A. Standish<ref name="Standish1975"/> was essentially a post mortem. The [[Forth (programming language)|Forth]] was an exception, but it went essentially unnoticed.

=== Character of the historical movement ===
As typically envisioned, an extensible language consisted of a base language providing elementary computing facilities, and a [[metalanguage]] able to modify the base language. A program then consisted of metalanguage modifications and code in the modified base language.

The most prominent language-extension technique used in the movement was macro definition. Grammar modification was also closely associated with the movement, resulting in the eventual development of [[adaptive grammar]] [[Formalism (philosophy of mathematics)|formalisms]]. The [[Lisp (programming language)|Lisp]] language community remained separate from the extensible language community, apparently because, as one researcher observed,

<blockquote>any programming language in which programs and data are essentially interchangeable can be regarded as an extendible [sic] language. ... this can be seen very easily from the fact that Lisp has been used as an extendible language for years.<ref name="Harrison1969">Harrison, M.C., in "Panel on the Concept of Extensibility", pp. 53–54 of the 1969 symposium.</ref></blockquote>

At the 1969 conference, [[Simula]] was presented as an extensible language.

Standish described three classes of language extension, which he named ''[[paraphrase]]'', ''orthophrase'', and ''metaphrase'' (otherwise paraphrase and metaphrase being [[translation]] terms).

* [[Paraphrase]] defines a facility by showing how to exchange it for something formerly defined (or to be defined). As examples, he mentions macro definitions, ordinary procedure definitions, grammatical extensions, data definitions, operator definitions, and control structure extensions.
* Orthophrase adds features to a language that could not be achieved using the base language, such as adding an [[input/output]] (I/O) system to a base language formerly with no I/O primitives. Extensions must be understood as orthophrase ''relative'' to some given base language, since a feature not defined in terms of the base language must be defined in terms of some other language. This corresponds to the modern notion of [[Plug-in (computing)|plug-ins]].
* Metaphrase modifies the interpretation rules used for pre-existing expressions. This corresponds to the modern notion of [[reflective programming]] (reflection).

=== Death of the historical movement ===
Standish attributed the failure of the extensibility movement to the difficulty of programming successive extensions. A programmer might build a first shell of macros around a base language. Then, if a second shell of macros is built around that, any subsequent programmer must be intimately familiar with both the base language, and the first shell. A third shell would require familiarity with the base and both the first and second shells, and so on. Shielding a programmer from lower-level details is the intent of the [[Abstraction (computer science)|abstraction]] movement that supplanted the extensibility movement.

Despite the earlier presentation of Simula as extensible, by 1975, Standish's survey does not seem in practice to have included the newer abstraction-based technologies (though he used a very general definition of extensibility that technically could have included them). A 1978 history of programming abstraction from the invention of the computer until then, made no mention of macros, and gave no hint that the extensible languages movement had ever occurred.<ref name="Guarino1978">Guarino, L.R., "[https://cds.cern.ch/record/119689 The Evolution of Abstraction in Programming Languages]", ''CMU-CS-78-120'', Department of Computer Science, Carnegie-Mellon University, Pennsylvania, 22 May 1978.</ref> Macros were tentatively admitted into the abstraction movement by the late 1980s (perhaps due to the advent of [[hygienic macros]]), by being granted the pseudonym ''syntactic abstractions''.<ref name="Gabriel1989">Gabriel, Richard P., ed., "[https://dl.acm.org/citation.cfm?id=66092 Draft Report on Requirements for a Common Prototyping System]", ''SIGPLAN Notices'' 24 no. 3 (March 1989), pp. 93ff.</ref>