Editing Extensible programming (section)

=== 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).