syntax error - Lisp, instructions not working in defun

Question

This question already has answers here:

Answer 1

What you are missing is that in Lisp parentheses are meaningful.

In C/Java/Python &c, the following expressions are the same:

a+b
(a+b)
(a)+(b)
(((a)+(b)))
(((((a)+(b)))))

In Lisp, the following expressions are very different:

• a --- a symbol
• (a) --- a list with a single element, which is the symbol a
• (1 (2)) --- a list of two elements:
  1. number 1
  2. list of of length 1, containing number 2

In your case, function (note indentation and paren placement!)

(defun prefixToInfix (x)
  ((if (listp x) (list (second x) (first x) (first (last x))) x)))

has extra parens around if (and this causes the whole if form to be interpreted as a function, with disastrous results), and should be (note line breaks and indentation - lispers do not count parens, they look at indentation to understand the code, see http://lisp-lang.org/style-guide/)

(defun prefix-to-infix (x)
  (if (listp x)
      (list (second x)
            (first x)
            (first (last x)))
      x))

PS. See also recommendations in want to learn common lisp.