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11 posts · 8502 views
(defun new (&optional value)
(cons value nil))
(defun value-of (ptr)
(car ptr))
(defun (setf value-of) (new-value ptr)
(setf (car ptr) new-value))
;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;
(defun blah (x)
(setf (value-of x) 4321))
(defun test ()
(let ((my-ptr (new 1234)))
(blah my-ptr)
(value-of my-ptr)))
;; (test) => 4321LispProgrammer wrote:if I have a function like....In Lisp, you can't pass the variable by reference, you can only pass the value (which might itself be a reference). You can wrap the variable in a structure or in a cons cell, or you can pass the setter function.
(defun hello (x)
(setf x 6)
)
and then I ran...
(setf y 3)
(hello y)
How can I make it so that so that the value of y is changed to 6???
;; Implementation of reference-to-variable
(defmacro make-reference (x)
(let ((y (gensym)))
`(cons (lambda () ,x)
(lambda (,y) (setf ,x ,y)))))
(defun dereference (ref) (funcall (car ref)))
(defun (setf dereference) (value ref) (funcall (cdr ref) value))
(defun hello (x)
(setf (dereference x) 6))
(let ((y 3))
(hello (make-reference y))
(print y))
=> 6LispProgrammer wrote:if I have a function like....Why do you want to do this? How about:
(defun hello (x)
(setf x 6)
)
and then I ran...
(setf y 3)
(hello y)
How can I make it so that so that the value of y is changed to 6???
(defun hello (x) (* x 6)) ; For example
(setf x (hello x))(hello (+ 1 2))
(hello 3)
(mapcar #'hello (list 1 2 3 4))(defun hello (name)
(set name 6))
=> HELLO
(defun test ()
(let ((*a* 4))
(declare (special *a*))
(hello '*a*)
*a*))
=> TEST
(test)
=> 6Paul Donnelly wrote:And what if the argument you pass to HELLO isn't a variable?The most intuitive thing to do is to treat the argument as the «place» (generalized reference, see http://www.lispworks.com/documentation/ ... /05_aa.htm). E.g., (hello (car x)) should change the car of x, (hello (slot-value some-object 'some-slot)) should change the slot of the object.
dmitry_vk wrote:What when the argument is the result of a more complex computation, or is constant, or is supplied by a higher-order function rather than explicitly by the programmer? My point is that while OP might like modifying the argument, it's less generally useful than the way it is normally done. Of course if you do have a good reason, places are fine for their purpose.Paul Donnelly wrote:And what if the argument you pass to HELLO isn't a variable?The most intuitive thing to do is to treat the argument as the «place» (generalized reference, see http://www.lispworks.com/documentation/ ... /05_aa.htm). E.g., (hello (car x)) should change the car of x, (hello (slot-value some-object 'some-slot)) should change the slot of the object.
But the same syntax is impossible to use (unless hello is a macro), so argument should be wrapped into some other form (that creates the reference).
LispProgrammer wrote:This operates on a place, so it would be a macro:How can I make it so that so that the value of y is changed to 6???(defun hello (x) (setf x 6)) (setf y 3) (hello y)
CL-USER> (defmacro setf-6 (place) `(setf ,place 6))
SETF-6
CL-USER> (let ((x 3))
(setf-6 x)
x)
6LispProgrammer wrote:How can I make it so that so that the value of y is changed to 6???If you're doing this because you want to return multiple values (that being a common use case for pass-by-reference in C++), try (values) instead.
(let ((y 6))
(defun reset-y ()
(setf y 6))
(defun set-y (x)
(setf y x))
(defun get-y ()
y))