Lisp in Small Parts

Generalising Procedures 

In Processing Items in a List we saw how to perform an operation on every element in a list with:

(defun double-list (numbers)
  (if (null numbers) nil
    (cons (* 2 (first numbers)) (double-list (rest numbers)))))

Suppose we want to generalise this so we can perform any action, not just doubling, on each of the elements in the list. We would like to be able to create a procedure apply-list so we could write:

(defun double (n) (* 2 n))

(apply-list 'double '(2 3 5 7 11 13 17 19))

to double the numbers, or:

(apply-list 'evenp '(2 3 5 7 11 13 17 19))

to set each number to t if it's even.

To do this we can use a new procedure funcall, that calls a chosen procedure with the arguments we specify:

Calling a procedure: funcall

The procedure funcall (for function call) takes one or more parameters. The first parameter should evaluate to a procedure, which is called with the remaining parameters as its parameters. So, for example:

CL-USER > (funcall '+ 2 3)
5

sum-listin the previous section:

(defun apply-list (function items)
  (if (null items) nil
    (progn
      (funcall function (first items))
      (apply-list function (rest items)))))

We can test it out with the procedure print like this:

CL-USER > (apply-list 'print '(2 3 4 5 6))
2 
3 
4 
5 
6 
NIL

Applying a function to make a new list: change-list

As a second example we can write a generalised version of double-list, defined in Processing Items in a List, to create a general-purpose version called change-list:

(defun change-list (function items)
  (if (null items) nil
    (cons (funcall function (first items)) (change-list function (rest items)))))

Let's test it:

CL-USER > (change-list 'double '(2 3 5 7 11 13 17 19))
(4 6 10 14 22 26 34 38)

CL-USER > (change-list 'evenp '(2 3 5 7 11 13 17 19))
(T NIL NIL NIL NIL NIL NIL NIL)

(repeat-for 4 7 'print)

4 
5 
6 
7 

(combine '+ '(1 2 3 4))

(combine '* '(1 2 3 4))