Re: Newbie problem: naked left-hand reference

On 4/19/07, Kjetil S. Matheussen <[email protected]> wrote:
> On Thu, 19 Apr 2007, Jon Kleiser wrote:
>
...
> is another way of writing
>
> (let loop ((i m))
>    (letrec ((i2 (* i i))
>            (ds (desc-series i2)))
>      ...))
>
> which gives undefined behaviour because you shouldn't reference already
> defined variables in variables in a letrec block. (Hmm, that was probably
> a bad explanation). Anyway, the reference to "i2" is the problem, since
> its defined elsewhere in the letrec.

I do not really understand the explanation.

Here is a way to solve the problem (and better style, IMO):

(define do-from-to
  (lambda (m n)
    (let loop ((i m))
      (let* ((i2 (* i i))
             (ds (desc-series i2)))
        (display ds)
        (newline)
        (if (= i n) 'done
            (loop (+ i 1)))))))

Best Regards,

Victor Rodriguez.

>
>
> > I also have another question:
> > What's the easiest way to meassure how long a part of a sisc program takes
> > to execute?
> >
>
> The time function, perhaps... I use this macro:
>
> (define-macro (c-time expr)
>    `(cadr (time (eval ',expr))))
>
> (c-time (do-from-to 2 4))
>
>
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