Wouldn't this technique have trouble with code movement issues? For example, if I have
(iter (for x in data-table)
   (averaging x into mean)
   (finally (return mean)))
I know that's a contrived example, but I think the point is clear: if I'm going to make `averaging` a collector, I'd like to be able to use its result, and if that result is computed in a `finally` block that's invisible and out of my control, I can't do it, can I? There's `finally-protected`, but I think that's the *opposite* of what I need.


On 13 Mar 2018, at 10:08, Russ Tyndall wrote:

Here is an existing "sampling" clause to pull a random sample from a larger data set.  The long and short is just use a finally clause, as you would when writing a normal iterate loop.

(iterate:defmacro-clause (sampling expr &optional into var size size)
  "resevoir sample the input"
  (let ((sample (or var iterate::*result-var*)))
    (alexandria:with-unique-names (i sample-size sigil buffer row)
        (with ,sample)
        (with ,sample-size = (or ,size 100))
        (with ,buffer = (make-array ,sample-size :initial-element ',sigil))
        (with ,i = 0)
        (if (< ,i ,sample-size)
            (setf (aref ,buffer ,i) ,expr)
            (let ((r (random ,i)))
              (when (< r ,sample-size)
                (setf (aref ,buffer r) ,expr))))
        (incf ,i)
         ;; convert our sample to a list, but only if we actually took the sample
         (when (plusp ,i)
           (setf ,sample
                 (iter (for ,row in-vector ,buffer)
                   (until (eq ,row ',sigil))
                   (collect ,row)))))))))

Russ Tyndall
On 03/13/2018 10:49 AM, Robert Goldman wrote:

I was going to define an |AVERAGING| collector clause for iterate, but I'm not sure how to do it. The obvious thing, it seemed to me, would be to sum the values as I go along, and count them, and then divide the sum by the count when leaving the loop.

But the examples for |DEFMACRO-CLAUSE| in the manual do all of their work while iterating, and there doesn't seem to be an "at-end" hook. Is the kind of thing I would like feasible, and if so, how is it to be done?


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