That's easy. You only need to change the list representation. All you need is a Lisp interpreter.
The Common Lisp implementation LispWorks provides us with a Lisp interpreter:
CL-USER 137 > (defun factorial (n)
(if (<= n 1)
1
(* n (factorial (- n 1)))))
FACTORIAL
CL-USER 138 > (fifth (function-lambda-expression #'factorial))
(IF (<= N 1) 1 (* N (FACTORIAL (- N 1))))
CL-USER 139 > (fourth (fifth (function-lambda-expression #'factorial)))
(* N (FACTORIAL (- N 1)))
CL-USER 140 > (setf (first (fourth (fifth (function-lambda-expression
#'factorial))))
'+)
+
CL-USER 141 > (fourth (fifth (function-lambda-expression #'factorial)))
(+ N (FACTORIAL (- N 1)))
CL-USER 142 > (factorial 10)
55
CL-USER 143 > (setf (first (fourth (fifth (function-lambda-expression
#'factorial))))
'*)
*
CL-USER 144 > (factorial 10)
3628800
Here is an example where a function modifies itself. To make it slightly easier, I use a feature of Common Lisp: it allows me to write code which is not just some nested list, but a graph. In this case the function can access its own code:
CL-USER 180 > (defun factorial (n)
(if (<= n 1)
1
(progn
(setf (first '#1=(* n (factorial (- n 1))))
(case (first '#1#)
(+ '*)
(* '+)))
#1#)))
FACTORIAL
Above function alternatively uses +
or *
by modifying its code.
#1=
is a label in the expression, #1#
then references that label.
CL-USER 181 > (factorial 10)
4555
In earlier times (70s/80s) in some Lisp groups the developers were not using a text editor to write Lisp code, but a structure editor. The editor commands were directly changing the structure of the Lisp code.