Category:TXR: Difference between revisions
→What's with all that @ stuff?: Revise with up-to-date info.
(→What's with all that @ stuff?: Correction.) |
(→What's with all that @ stuff?: Revise with up-to-date info.) |
||
Line 27:
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". Originally,
The second unusual feature in TXR is that the "tokens" in the pattern matching language are essentially themselves Lisp symbols and expressions. These "tokens" are used to create a block-structured language. This is quite odd. For instance a construct might begin with a <code>@(collect :vars (foo))</code>. This is a Lisp expression with interior structure, but to the parser of the pattern language, it's also basically just a token, like a giant keyword.
=== Dual Personality ===
| |||