Question

I am reading through a book for homework, and I understand that using #’ is treating the variable as a function instead of a variable. But I am a little hazy on FUNCALL. I understand that lisp makes object out of variables, so is the function name just a ‘pointer’ (may be a bad word, but hopefully you get what I mean), in which case you use #’ to invoke it, or is funcall the only way to invoke them? ex.

(defun plot (fn min max step)
(loop for i from min to max by step do
      (loop repeat (funcall fn i) do (format t "*"))
      (format t "~%")))

couldn’t I just do:

(defun plot (fn min max step)
(loop for i from min to max by step do
      (loop repeat #'(fn i) do (format t "*"))
      (format t "~%")))

I guess my confusion lies in what exactly is in the function names. When I read the book, it said that the variable’s value is what will be the function object.

Answer 1

#'function-name is (function function-name). Nothing is called, evaluating either results in the function associated with function-name (the object representing the function). funcall is used to call functions.

See funcall and function in the HyperSpec.

Sample session using both:

CL-USER> (defun square (x) (* x x))
SQUARE
CL-USER> #'square
#<FUNCTION SQUARE>
CL-USER> (function square)
#<FUNCTION SQUARE>
CL-USER> (funcall #'square 3)
9
CL-USER> (funcall 'square 3)
9

The second invocation of funcall works because it also accepts a symbol as function designator (see the link for funcall above for details).