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POCommon Lisp Backquote/Backtick: How to Use?
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  1. POCommon Lisp Backquote/Backtick: How to Use?
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    copied!<p>I am having trouble with Lisp's backquote read macro. Whenever I try to write a macro that seems to require the use of embedded backquotes (e.g., <code>``(w ,x ,,y)</code> from Paul Graham's <em>ANSI Common Lisp</em>, page 399), I cannot figure out how to write my code in a way that compiles. Typically, my code receives a whole chain of errors preceded with "Comma not inside a backquote." Can someone provide some guidelines for how I can write code that will evaluate properly?</p> <p>As an example, I currently need a macro which takes a form that describes a rule in the form of <code>'(function-name column-index value)</code> and generates a predicate lambda body to determine whether the element indexed by <code>column-index</code> for a particular row satisfies the rule. If I called this macro with the rule <code>'(&lt; 1 2)</code>, I would want a lambda body that looks like the following to be generated:</p> <pre><code>(lambda (row) (&lt; (svref row 1) 2)) </code></pre> <p>The best stab I can make at this is as follows:</p> <pre><code>(defmacro row-satisfies-rule (rule) (let ((x (gensym))) `(let ((,x ,rule)) (lambda (row) (`,(car ,x) (svref row `,(cadr ,x)) `,(caddr ,x)))))) </code></pre> <p>Upon evaluation, SBCL spews the following error report:</p> <pre><code>; in: ROW-SATISFIES-RULE '(&lt; 1 2) ; ((CAR #:G1121) (SVREF ROW (CADR #:G1121)) (CADDR #:G1121)) ; ; caught ERROR: ; illegal function call ; (LAMBDA (ROW) ((CAR #:G1121) (SVREF ROW (CADR #:G1121)) (CADDR #:G1121))) ; ==&gt; ; #'(LAMBDA (ROW) ((CAR #:G1121) (SVREF ROW (CADR #:G1121)) (CADDR #:G1121))) ; ; caught STYLE-WARNING: ; The variable ROW is defined but never used. ; (LET ((#:G1121 '(&lt; 1 2))) ; (LAMBDA (ROW) ((CAR #:G1121) (SVREF ROW (CADR #:G1121)) (CADDR #:G1121)))) ; ; caught STYLE-WARNING: ; The variable #:G1121 is defined but never used. ; ; compilation unit finished ; caught 1 ERROR condition ; caught 2 STYLE-WARNING conditions #&lt;FUNCTION (LAMBDA (ROW)) {2497F245}&gt; </code></pre> <p>How can I write macros to generate the code I need, and in particular, how do I implement <code>row-satisfies-rule</code>?</p> <hr> <p>Using the ideas from Ivijay and discipulus, I have modified the macro so that it compiles and works, even allowing forms to be passed as the arguments. It runs a bit differently from my originally planned macro since I determined that including <code>row</code> as an argument made for smoother code. However, it is ugly as sin. Does anyone know how to clean it up so it performs the same without the call to <code>eval</code>?</p> <pre><code>(defmacro row-satisfies-rule-p (row rule) (let ((x (gensym)) (y (gensym))) `(let ((,x ,row) (,y ,rule)) (destructuring-bind (a b c) ,y (eval `(,a (svref ,,x ,b) ,c)))))) </code></pre> <p>Also, an explanation of clean, Lispy ways to get macros to generate code to properly evaluate the arguments at runtime would be greatly appreciated.</p>
 

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