Category:TXR: Difference between revisions
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In TXR Lisp, the <code>@</code> character has more "meta" piled on top of it: <code>@foo</code> denotes <code>(sys:var foo)</code>, and <code>@(foo ...)</code> denotes <code>(sys:expr foo>)</code>. In any context which needs to separate meta-variables and meta-expressions from variables and expressions, this may come in handy. It's used by the <code>op</code> operator for currying, for instance <code>(op * @1 @1)</code> returns a function of one argument which returns the square of that argument. The implementation of the operator looks for syntax like <code>(sys:var 1)</code> and replaces it with the arguments of the generated lambda function. The <code>@</code> character also appears in quasiliteral strings, where it interpolates the values of variables and expressions as text.
TXR is somewhat unusual in that the relationship between a domain-specific language (DSL) and general-purpose host language is reversed. Typically, at least in Lisp systems, DSL's are embedded into the parent language. In TXR, the "outer shell" is the domain-specific language for extracting text, and Lisp is embedded in it as "computational appliance". It doesn't take much to reach the Lisp though: a TXR source file can just consist of a single <code>@(do ...)</code> directive which contains nothing but TXR Lisp forms. Also, TXR Lisp evaluation is available from program invocation via the <code>-e</code> and <code>-p</code> options.
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