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3 points by waterhouse 3367 days ago | link | parent

Given that you already iterate through the list, it can be made cheap to get the last item:

  (def prl args ;returns last arg
    (let u args
      (while cdr.u
        (pr car.u)
        (zap cdr u))
      (pr car.u)))
  arc> (prl 1 2 3)
  1233
Even though 'u points to the list 'args, I can modify 'u to point to another part of the list, and the list itself is not modified. I thus traverse the list only once here.

Also, note that the REPL prints the return value on the same line as whatever other output. I think this is annoying. Common Lisp has a 'fresh-line procedure that prints a newline to an output-stream if and only if at least one character has been printed to that stream and the last character printed was not a newline. It would be nice to use that in the toplevel procedure.

Having written that, I looked at the PLT docs and figured out how to at least tell whether nothing has been written to an output-port since the last time you checked, and I hacked the toplevel function in ac.scm to print a newline when the expression printed something, whether or not that something ended with a newline. I'm not sure whether I like this better:

  arc> "ach"
  "ach"
  arc> (pr "ach")
  ach
  "ach"
  arc> (prn "ach")
  ach
  
  "ach"
Changes in ac.scm (I haven't learned to use diff, so I'll record them like this):

  ;Relevant part of resulting definition of tl2 in ac.scm:
        (if (eqv? expr ':a)
            'done
            (let ((n (next-char-place))
                  (val (arc-eval expr)))
              (if (< n (next-char-place))
                  (newline))
              (write (ac-denil val))

  ;Then add this:
  (define (next-char-place)
    (let-values (((a b n) (port-next-location (current-output-port))))
      n))