Question

(1)Create a LISP function FLATTEN, a function that returns all the elements of an arbitrarily nested list in a single-level list.

(FLATTEN ’((A B (R)) A C (A D ((A (B)) R) A))) should return (A B R A C A D A B R A).

(2) Create a Lisp function EXP-EVAL, a function that evaluates an arithmetic expression. You may assume that the binary operators used for an arithmetic expression are: +, -, *, and /, and each of the (nested) expression is well-formed (parenthesized) binary expression recursively. You may assume: each list is recursively well-formed with binary operator only (infix notation).

For example, (EXP-EVAL '(2 + (3 * 5))) should return 17.

Answer 1

`push` changes the symbol binding in scope. Thus the recursion `(rflatten el acc)` has it's own `acc` which is the result there but you don't do anything with the returned result and it doesn't alter the callee `acc`.

Perhaps a `(setf acc (rflatten el acc))` would fix that:

    (defun flatten (lst)
      (labels ((rflatten (lst1 acc)
                 (dolist (el lst1)
                   (if (listp el)
                       (setf acc (rflatten el acc))
                       (push el acc)))
                 acc))
        (reverse (rflatten lst nil))))