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. |
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. |
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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". |
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, TXR recognized only one kind of source file: one written in the TXR language. Modern TXR recognizes files with <code>.txr</code> and </code>.tl</code> suffixes. <code>.tl</code> files contain just TXR Lisp. "Hash bang" interpreter programs can be written in either language, without a suffix on the executable script file. TXR Lisp expression evaluation is also available from program invocation via the <code>-e</code> and <code>-p</code> options. |
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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. |
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. It begins a collect clause, and is followed by some optional material which may just be literal text, and must be terminated by the <code>@(end)</code> directive, which is another token-expression entity. |
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=== Dual Personality === |
=== Dual Personality === |
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