Question

I need to write a function in lisp with two arguments – list of argumentless functions and list of integers.
I need to evaluate functions from first list in order given by a second, i.e. (fun ‘(#’a #’b #’c) ‘(2 0 1)) should evaluate c, a, b.
I tried such function:

(defun z4(funs kols)
    (funcall (nth (first kols) funs))
    (z4 funs (rest kols))
)

but in funcall I am geting error

NIL is not of type CONS.

What does it means? I am getting same error by calling simply

(funcall (first funs))

so I assume it is something with with getting function from the list of functions. How can I evaluate function get from list of functions?

Answer 1

With each recursive call you reduce the kols parameter untill it becomes nil. When kols becomes nil you should terminate the recursion, so you should add the test for the terminating condition (i.e., for empty list):

(defun foo (funcs order)
  (unless (endp order)
    (funcall (nth (first order) funcs))
    (foo funcs (rest order))))

I suggest a more readable solution (it’s also more preferrable since ANSI Common Lisp Standard doesn’t force implementations to perform tail call optimization):

(defun foo (funcs order)
  (loop for n in order do (funcall (nth n funcs))))

In the previous examples I assume that you run your functions for their side effects, not for the return values.

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Edit 1

As Vatine noted, using nth and lists to provide a collection with random access is not good for the performance, so it might be worth to store functions in a vector (that is a one-dimensional array) rathen than in a list. So, assuming that funcs is a vector of functions, the function can be defined as follows:

(defun foo (funcs order)
  (loop for n in order do (funcall (aref funcs n))))

Moreover, we can replace aref with svref if the array of functions is a simple vector (glossary entry). The former is more general, but the latter may be faster in some implementations.

One can create a simple vector of functions using either

(vector #'func-a #'func-b ...)

or

#(func-a func-b ...)

A simple vector of functions can be created using the make-array function as well, but only if :adjustable and :fill-pointer keyword arguments are unspecified or nil.