Adding Common Lisp examples with .cl extension

samples/Common Lisp/macros-advanced.cl

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+;; @file macros-advanced.cl
+;;
+;; @breif Advanced macro practices - defining your own macros
+;;
+;; Macro definition skeleton:
+;; (defmacro name (parameter*)
+;;   "Optional documentation string"
+;;   body-form*)
+;;
+;; Note that backquote expression is most often used in the `body-form`
+;;
+
+; `primep` test a number for prime
+(defun primep (n)
+  "test a number for prime"
+  (if (< n 2) (return-from primep))
+  (do ((i 2 (1+ i)) (p t (not (zerop (mod n i)))))
+      ((> i (sqrt n)) p)
+    (when (not p) (return))))
+; `next-prime` return the next prime bigger than the specified number
+(defun next-prime (n)
+  "return the next prime bigger than the speicified number"
+  (do ((i (1+ n) (1+ i)))
+      ((primep i) i)))
+;
+; The recommended procedures to writting a new macro are as follows:
+; 1. Write a sample call to the macro and the code it should expand into
+(do-primes (p 0 19)
+  (format t "~d " p))
+; Expected expanded codes
+(do ((p (next-prime (- 0 1)) (next-prime p)))
+    ((> p 19))
+  (format t "~d " p))
+; 2. Write code that generate the hardwritten expansion from the arguments in
+; the sample call
+(defmacro do-primes (var-and-range &rest body)
+  (let ((var (first var-and-range))
+        (start (second var-and-range))
+        (end (third var-and-range)))
+    `(do ((,var (next-prime (- ,start 1)) (next-prime ,var)))
+         ((> ,var ,end))
+      ,@body)))
+; 2-1. More concise implementations with the 'parameter list destructuring' and
+; '&body' synonym, it also emits more friendly messages on incorrent input.
+(defmacro do-primes ((var start end) &body body)
+  `(do ((,var (next-prime (- ,start 1)) (next-prime ,var)))
+       ((> ,var ,end))
+    ,@body))
+; 2-2. Test the result of macro expansion with the `macroexpand-1` function
+(macroexpand-1 '(do-primes (p 0 19) (format t "~d " p)))
+; 3. Make sure the macro abstraction does not "leak"
+(defmacro do-primes ((var start end) &body body)
+  (let ((end-value-name (gensym)))
+    `(do ((,var (next-prime (- ,start 1)) (next-prime ,var))
+          (,end-value-name ,end))
+         ((> ,var ,end-value-name))
+      ,@body)))
+; 3-1. Rules to observe to avoid common and possible leaks
+;   a. include any subforms in the expansion in positions that will be evaluated
+;      in the same order as the subforms appear in the macro call
+;   b. make sure subforms are evaluated only once by creating a variable in the
+;      expansion to hold the value of evaluating the argument form, and then
+;      using that variable anywhere else the value is needed in the expansion
+;   c. use `gensym` at macro expansion time to create variable names used in the
+;      expansion
+;
+; Appendix I. Macro-writting macros, 'with-gensyms', to guranttee that rule c
+; gets observed.
+; Example usage of `with-gensyms`
+(defmacro do-primes-a ((var start end) &body body)
+  "do-primes implementation with macro-writting macro 'with-gensyms'"
+  (with-gensyms (end-value-name)
+    `(do ((,var (next-prime (- ,start 1)) (next-prime ,var))
+          (,end-value-name ,end))
+         ((> ,var ,end-value-name))
+      ,@body)))
+; Define the macro, note how comma is used to interpolate the value of the loop
+; expression
+(defmacro with-gensyms ((&rest names) &body body)
+  `(let ,(loop for n in names collect `(,n (gensym)))
+    ,@body)
+)