Collecting without LOOP

Published , 5 minute read

LOOP is a famously divisive iteration DSL. Personally, I love it. Iteration is central to programming, I don’t have any scruples about imperative code, and I already know a million weird languages, so what’s one more. Plus, it’s a zero cost abstraction and an awesome example of what a Lisp macro is capable of.

But I digress. In this post, I want to talk specifically about the collect keyword of LOOP: how it works, and how we can make our own version of it that’s useful in situations LOOP isn’t desirable or otherwise applicable.

Collecting 101

Consider the following example that evaluates to the list (2 4 6 8 10):

(loop for i from 1 to 10
      when (evenp i)
      collect i)

It counts from 1 to 10, and when an even number is encountered, that value is appended to a list and that list is returned. The LOOP syntax makes it all look pretty simple, but just beneath the syntax some fanciness is happening.

This fanciness is necessary because it’s not efficient to append to a linked list made of cons cells. You have to find the last cell in order to mutate it, and finding the last cell means traversing all the cells that precede it. The longer the list is, the longer it will take.

The way around this problem — the fanciness — is to maintain both the head and the tail of the list you’re appending to. When you want to append to the list, you CONS up a new tail. When you’re done appending, you return the head.

The following example also returns the list (2 4 6 8 10) but uses a simplified form of LOOP and maintains the result list manually. It’s kind of hairy, but don’t worry, we’ll clean it up shortly:

(let ((head nil)
      (tail nil)
      (i 1))
   (when (> i 10) (return head))
   (when (evenp i)
     (if (null tail)
         (setq tail (cons i nil)
               head tail)
       (let ((new-tail (cons i nil)))
         (setf (cdr tail) new-tail
               tail new-tail))))
   (incf i)))

Here’s a line-by-line breakdown of what’s going on:

Encapsulated collecting

It wouldn’t be fun to have to write that head/tail management code all the time, which is one reason that LOOP and its collect keyword are so useful. But collecting doesn’t have to be so wordy, even without LOOP.

One way to make collecting more accessible is to wrap it in a function-based interface. Consider the following function, COLLECTOR:

(defun collector ()
  "Maintains the head of a list and returns a function that appends an
item to the list. When the returned function is called with no
arguments, returns the head."
  (let ((tail nil)
        (head nil))
    (lambda (&optional (item nil argument?))
        ((not argument?) head)
        ((null tail) (setq tail (cons item nil)
                           head tail))
        (t (let ((new-tail (cons item nil)))
             (setf (cdr tail) new-tail
                   tail new-tail)))))))

COLLECTOR encapsulates the collecting logic in the form of a function that returns a function. The returned function can be called two ways:

  1. With an argument, which causes the argument to be collected
  2. Without an argument, which returns the head of the list containing collected items

Here’s some example usage:

(setq collect (collector))
(funcall collect 1)
(funcall collect 2)
(funcall collect 3)
(funcall collect) ;=> (1 2 3)

Note that FUNCALL is necessary here because we have stored the function returned by COLLECTOR into a variable and Common Lisp is a Lisp-2.

Our code for collecting even numbers gets a lot clearer with COLLECTOR:

(let ((collect (collector))
      (i 1))
    ((> i 10) (return (funcall collect)))
    ((evenp i) (funcall collect i)))
   (incf i)))

We can arguably simplify even further by using the other iteration constructs DO or DOTIMES:

;; with DO
(do ((collect (collector))
     (i 1 (1+ i)))
    ((> i 10) (funcall collect))
  (when (evenp i)
    (funcall collect i)))

;; with DOTIMES
(let ((collect (collector)))
  (dotimes (i 10 (funcall collect))
    (let ((i (1+ i)))
      (when (evenp i)
        (funcall collect i)))))

Other observations and considerations