Question

I’m very new to Scheme and I’m getting an “ill-formed special form” error when I try to use this function. I want the function to return left if the length of original is 0. Else I want it to execute the calls to append and the recursive call. I imagine I’ve probably done something really silly that is causing this. ANy help?

(define (partition left right original)
(if (= (length original) 0) left
    (append left (car original))
        (append right (car (reverse original)))
              (partition left right (reverse original))))

Answer 1

Before I begin, do try to indent your programs properly. In this case, it would have made the error much more obvious.

You’re running into a speedbump that many Scheme newbies hit coming from other languages:

if in Lisp (including scheme) is actually the ternary operator, not the if block from the C-like world.

Your code tries to present if with more than three forms, so it fails. Naively, what you could do is use the block construct begin (progn in Common Lisp) to make sure that a single branch of the conditional only contains a single form.

(define (partition left right original)
  (if (= (length original) 0)
      left
      (begin 
        (append left (car original))
        (append right (car (reverse original)))
        (partition left right (reverse original))))) 

Now, I get the feeling that this won’t do what you want either, because append is functional (that is, it returns a result without modifying its arguments). In other words, neither of your calls to append will do anything, and original will never get shorter. What you actually seem to want here is something like

(define (partition left right original)
  (if (= (length original) 0)
      left
      (partition (cons (car original) left)
                 (cons (last original) right)
                 (cdr (take original (- (length original) 1))))))