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comprehension.lisp

 1 ; Copyright (c) 2008 the authors listed at the following URL, and/or
 2 ; the authors of referenced articles or incorporated external code:
 3 ; http://en.literateprograms.org/List_comprehension_(Lisp)?action=history&offset=20060908174651
 4 ; 
 5 ; Permission is hereby granted, free of charge, to any person obtaining
 6 ; a copy of this software and associated documentation files (the
 7 ; "Software"), to deal in the Software without restriction, including
 8 ; without limitation the rights to use, copy, modify, merge, publish,
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10 ; permit persons to whom the Software is furnished to do so, subject to
11 ; the following conditions:
12 ; 
13 ; The above copyright notice and this permission notice shall be
14 ; included in all copies or substantial portions of the Software.
15 ; 
16 ; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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22 ; SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
23 ; 
24 ; Retrieved from: http://en.literateprograms.org/List_comprehension_(Lisp)?oldid=7599
25 
26 
27 (defun make-loops (var-list-pairs last-loop-body)
28   (if (endp var-list-pairs)
29     last-loop-body
30     (destructuring-bind ((var list) . therest) var-list-pairs
31       `(loop for ,var in ,list do ,(make-loops therest last-loop-body)))))
32 
33 
34 (defmacro make-comprehension (expr vars-in-lists guards)
35   (let ((accum (gensym)))
36     `(let ((,accum (list)))
37        (progn 
38 	 ,(make-loops vars-in-lists
39 		      `(if (and ,@guards)
40 			 (push ,expr ,accum)))
41 	 (nreverse ,accum)))))
42


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