Problem Statement

Create a list of ten functions, in the simplest manner possible   (anonymous functions are encouraged),   such that the function at index   i   (you may choose to start   i   from either   0   or   1),   when run, should return the square of the index,   that is,   i 2. Display the result of running any but the last function, to demonstrate that the function indeed remembers its value.

Demonstrate how to create a series of independent closures based on the same template but maintain separate copies of the variable closed over. In imperative languages, one would generally use a loop with a mutable counter variable. For each function to maintain the correct number, it has to capture the value of the variable at the time it was created, rather than just a reference to the variable, which would have a different value by the time the function was run. See also: Multiple distinct objects

Let's start with the solution:

CL-USER> (defparameter alist
	   (loop for i from 1 to 10
	      collect (cons i (let ((i i))
				(lambda () (* i i))))))
ALIST
CL-USER> (funcall (cdr (assoc 2 alist)))
4
CL-USER> (funcall (cdr (assoc 8 alist)))
64