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XLR: Extensible Language and Runtime
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Historically, most programming languages have been defined by a specific syntax, semantics, and a predefined set of abstraction mechanisms (functions, objects, keywords, composition rules). Consequently, there are always some concepts that are not easily represented with such languages. For instance, it is difficult to integrate symbolic differentiation, Prolog-style backtracking or Ada-style tasking into languages such as C or Java. Many simple concepts, such as maximum, are actually quite difficult to represent using most existing languages. More precisely, it is almost always possible to represent any concept in any language. It doesn't take much to be able to perform any computation. In assembler, you don't need more than the notion of addressable memory, loads, stores, and half a dozen basic operations. The Turing machine needed even less than that. But this too often requires incredibly tortuous mechanisms, or very verbose code where the intent is all but lost. Or, as Philip Greenspun once put it, "Any sufficiently complicated C or Fortran program contains an ad-hoc, informally-specified bug-ridden slow implementation of half of Common Lisp". Of course, nobody wants to code directly to a Turing machine, and most programmers try to avoid assembler. So the question becomes: are the limits of your programming languages visible to you? Do they reduce your productivity? This is a valid question, because the concepts being implemented in common programming languages are the most common ones. Practically all languages features building-blocks such as functions, user-defined data types, objects, and reasonable composition rules. These may be sufficient for your purpose. Yet most of these languages won't deal gracefully with concepts that fall outside of their domain of expertise, much like C doesn't deal gracefully with differentiation or backtracking. This is more visible with slightly less common problem space, where programmers often resort to domain specific languages, in other words languages tailored for a particular application. Even that is not always a solution, since developing a domain-specific language is complex, and quite often not worth the effort. Lowering this barrier to language extensions is an objective of XL.
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Copyright 2006 Christophe de Dinechin (Blog)
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