#### 3.8.9 define-compiler-macro [Macro]

`define-compiler-macro` name lambda-list [[{declaration}* | documentation]] {form}*
⇒ name

#### Arguments and Values::

name—a function name.

lambda-list—a macro lambda list.

declaration—a declare expression; not evaluated.

documentation—a string; not evaluated.

form—a form.

#### Description::

[Editorial Note by KMP: This definition probably needs to be fully expanded to not refer through the definition of defmacro, but should suffice for now.]

This is the normal mechanism for defining a compiler macro function. Its manner of definition is the same as for defmacro; the only differences are:

`*`

  The name can be a function name naming any function or macro.

`*`

  The expander function is installed as a compiler macro function for the name, rather than as a macro function.

`*`

  The &whole argument is bound to the form argument that is passed to the compiler macro function. The remaining lambda-list parameters are specified as if this form contained the function name in the car and the actual arguments in the cdr, but if the car of the actual form is the symbol funcall, then the destructuring of the arguments is actually performed using its cddr instead.

`*`

  Documentation is attached as a documentation string to name (as kind compiler-macro) and to the compiler macro function.

`*`

  Unlike an ordinary macro, a compiler macro can decline to provide an expansion merely by returning a form that is the same as the original (which can be obtained by using &whole).

#### Examples::

     (defun square (x) (expt x 2)) ⇒  SQUARE
     (define-compiler-macro square (&whole form arg)
       (if (atom arg)
           `(expt ,arg 2)
           (case (car arg)
             (square (if (= (length arg) 2)
                         `(expt ,(nth 1 arg) 4)
                         form))
             (expt   (if (= (length arg) 3)
                         (if (numberp (nth 2 arg))
                             `(expt ,(nth 1 arg) ,(* 2 (nth 2 arg)))
                             `(expt ,(nth 1 arg) (* 2 ,(nth 2 arg))))
                         form))
             (otherwise `(expt ,arg 2))))) ⇒  SQUARE
     (square (square 3)) ⇒  81
     (macroexpand '(square x)) ⇒  (SQUARE X), false
     (funcall (compiler-macro-function 'square) '(square x) nil)
    ⇒  (EXPT X 2)
     (funcall (compiler-macro-function 'square) '(square (square x)) nil)
    ⇒  (EXPT X 4)
     (funcall (compiler-macro-function 'square) '(funcall #'square x) nil)
    ⇒  (EXPT X 2)

     (defun distance-positional (x1 y1 x2 y2)
       (sqrt (+ (expt (- x2 x1) 2) (expt (- y2 y1) 2))))
    ⇒  DISTANCE-POSITIONAL
     (defun distance (&key (x1 0) (y1 0) (x2 x1) (y2 y1))
       (distance-positional x1 y1 x2 y2))
    ⇒  DISTANCE
     (define-compiler-macro distance (&whole form
                                      &rest key-value-pairs
                                      &key (x1 0  x1-p)
                                           (y1 0  y1-p)
                                           (x2 x1 x2-p)
                                           (y2 y1 y2-p)
                                      &allow-other-keys
                                      &environment env)
       (flet ((key (n) (nth (* n 2) key-value-pairs))
              (arg (n) (nth (1+ (* n 2)) key-value-pairs))
              (simplep (x)
                (let ((expanded-x (macroexpand x env)))
                  (or (constantp expanded-x env)
                      (symbolp expanded-x)))))
         (let ((n (/ (length key-value-pairs) 2)))
           (multiple-value-bind (x1s y1s x2s y2s others)
               (loop for (key) on key-value-pairs by #'cddr
                     count (eq key ':x1) into x1s
                     count (eq key ':y1) into y1s
                     count (eq key ':x2) into x2s
                     count (eq key ':y1) into y2s
                     count (not (member key '(:x1 :x2 :y1 :y2)))
                       into others
                     finally (return (values x1s y1s x2s y2s others)))
             (cond ((and (= n 4)
                         (eq (key 0) :x1)
                         (eq (key 1) :y1)
                         (eq (key 2) :x2)
                         (eq (key 3) :y2))
                    `(distance-positional ,x1 ,y1 ,x2 ,y2))
                   ((and (if x1-p (and (= x1s 1) (simplep x1)) t)
                         (if y1-p (and (= y1s 1) (simplep y1)) t)
                         (if x2-p (and (= x2s 1) (simplep x2)) t)
                         (if y2-p (and (= y2s 1) (simplep y2)) t)
                         (zerop others))
                    `(distance-positional ,x1 ,y1 ,x2 ,y2))
                   ((and (< x1s 2) (< y1s 2) (< x2s 2) (< y2s 2)
                         (zerop others))
                    (let ((temps (loop repeat n collect (gensym))))
                      `(let ,(loop for i below n
                                   collect (list (nth i temps) (arg i)))
                         (distance
                           ,@(loop for i below n
                                   append (list (key i) (nth i temps)))))))
                   (t form))))))
    ⇒  DISTANCE
     (dolist (form
               '((distance :x1 (setq x 7) :x2 (decf x) :y1 (decf x) :y2 (decf x))
                 (distance :x1 (setq x 7) :y1 (decf x) :x2 (decf x) :y2 (decf x))
                 (distance :x1 (setq x 7) :y1 (incf x))
                 (distance :x1 (setq x 7) :y1 (incf x) :x1 (incf x))
                 (distance :x1 a1 :y1 b1 :x2 a2 :y2 b2)
                 (distance :x1 a1 :x2 a2 :y1 b1 :y2 b2)
                 (distance :x1 a1 :y1 b1 :z1 c1 :x2 a2 :y2 b2 :z2 c2)))
       (print (funcall (compiler-macro-function 'distance) form nil)))
     |>  (LET ((#:G6558 (SETQ X 7))
     |>        (#:G6559 (DECF X))
     |>        (#:G6560 (DECF X))
     |>        (#:G6561 (DECF X)))
     |>    (DISTANCE :X1 #:G6558 :X2 #:G6559 :Y1 #:G6560 :Y2 #:G6561))
     |>  (DISTANCE-POSITIONAL (SETQ X 7) (DECF X) (DECF X) (DECF X))
     |>  (LET ((#:G6567 (SETQ X 7))
     |>        (#:G6568 (INCF X)))
     |>    (DISTANCE :X1 #:G6567 :Y1 #:G6568))
     |>  (DISTANCE :X1 (SETQ X 7) :Y1 (INCF X) :X1 (INCF X))
     |>  (DISTANCE-POSITIONAL A1 B1 A2 B2)
     |>  (DISTANCE-POSITIONAL A1 B1 A2 B2)
     |>  (DISTANCE :X1 A1 :Y1 B1 :Z1 C1 :X2 A2 :Y2 B2 :Z2 C2)
    ⇒  NIL

#### See Also::

compiler-macro-function , defmacro , documentation , Syntactic Interaction of Documentation Strings and Declarations

#### Notes::

The consequences of writing a compiler macro definition for a function in the `COMMON-LISP` package are undefined; it is quite possible that in some implementations such an attempt would override an equivalent or equally important definition. In general, it is recommended that a programmer only write compiler macro definitions for functions he or she personally maintains–writing a compiler macro definition for a function maintained elsewhere is normally considered a violation of traditional rules of modularity and data abstraction.