[hollerith]

None of the assignments my classmate struggled with required him write code that processes code, so I don't think that was the source of his confusion.

My guess is that it has more to do with the fact that CONS takes a whole list as an argument, then returns a different whole list, which superficially seems too computationally expensive to do in a loop.

[lispm]

> None of the assignments my classmate struggled with required him write code that processes code

That's not what I meant. What Lisp with s-expressions already makes more difficult is that the data syntax is used to express programs.

    (defun foo (list1 list2)
      (quote (defun foo (list1 list2) (list list1 list2))))

-> A symbol can be an identifier and just a piece of data.

Above, the first DEFUN is a standard identifier of the programming language Common Lisp. The second DEFUN is just a symbol.

The first DEFUN form is a program. The second DEFUN expression is just data. Thus it makes no sense to indent the second DEFUN expression like a program. We don't know if it is a program, it could be just anything.

-> A list can be data and a program.

This specific double nature of built-in data structures, here symbols and lists, is already a hurdle to get over.

The developer has to know which lists are data lists, which are forms and which are expressions in inside a form.

    (cond ((quote (a))))

->

    (cond ((quote (a)))) is a form, which is meant to be evaluated

    (a) is data

    ((quote (a)))  is a clause of COND, which is neither data, nor a form to be evaluated

Thus the user has to understand that in Lisp these lists serve different purposes. Which is which depends on the context and the Lisp syntax & evaluation rules.

This then even may make functions possible which contain their own definition as data:

    CL-USER 9 > #1=(defun foo (list1 list2)
                     (quote #1#))
    FOO

    CL-USER 10 > (foo '(1) '(2))
    #1=(DEFUN FOO (LIST1 LIST2) (QUOTE #1#))