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Understanding XL
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XLR: Extensible Language and Runtime
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XL is defined at three different levels, called XL0, XL1 and XL2. XL0 is a textual representation of an XL parse tree. It can be used to represent arbitrary data or code concepts.
// Example of valid XL0 input phone_book person "John Smith" phone "(555) 555-0123" address "1 rue des Pres Chevaux" city "Roquebrune sur Argens" zip 83520 person "Wolfgang Amadeus Mozart" city "Vienna" note "Not heard of him in a long while. Is he dead or what?" XL1 is one particular application of XL0 to represent an imperative programming language (in the lineage of ALGOL, Pascal, C, C++ or Ada). However, being based on XL0, that language is extensible. That language is implemented using the same translation mechanism used by compiler plug-ins. All XL1 features can be expected to be available in all XL implementations, though that is a rather small set (even basic arithmetic is actually in XL2). XL1 also defines basic language functionalities such as name look-up, modules, or expression evaluation.
// Example of valid XL1 fragment // This example is always valid because it doesn't assume a particular // form of arithmetic. Instantiating it with number=integer may require // integer arithmetic as typically provided by XL2 generic [type number] function Factorial(N : number) return number written N! assume N >= 0 is if N = 0 then return 1 else return N * (N-1)! XL2 adds a set of predefined libraries and language extensions on top of XL1. It implements most features that are now considered part of a decent language, from arithmetic to exception handling to file I/O to multi-tasking. Unlike XL1, the features in XL2 do not need to be present. For instance, a machine without floating-point capabilities doesn't need to implement floating-point arithmetic. However, programmers can rely on XL2 as a standard implementation of features. An implementation cannot provide an alternative approach to floating-point arithmetic if it doesn't also provide the XL2 standard implementation.
// Example of valid XL2 input // This example uses the example above, instantiated using integer arithmetic // It makes use of the standard console user interface (text I/O) import IO = XL.UI.CONSOLE to ShowFactorials is N : integer for N in 1..5 loop IO.WriteLn "The factorial of ", N, " is ", N!
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Copyright 2006 Christophe de Dinechin (Blog)
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