Re: Newbie problem: naked left-hand reference

On Thu, 19 Apr 2007, Jon Kleiser wrote:

> Thanks for explaining the "context for definition" thing! It helped me a
> lot. My first sisc program is now actually running, and it's quite fast.
> There is, however, one more error message that I've had that I don't quite
> understand. If I just remove a couple of expressions from my little sample
> program, so that it looks like this:
>
> (define desc-series
> 	(lambda (size)
> 		(let countdown ((n size) (s ()))
> 			(if (= n 0) s
> 				(countdown (- n 1) (append s (list n)))))))
>
> (define do-from-to
> 	(lambda (m n)
> 		(let loop ((i m))
> 			(begin
> 				(define i2 (* i i))
> 				(define ds (desc-series i2)) (display ds) (newline)
> 				(if (= i n) 'done
> 					(loop (+ i 1)))))))
>
> (do-from-to 2 4)
>
> Then, why do I get things like this:
>
> {warning: naked left-hand reference in letrec rhs: '%%_nNWd_9Tf51_i2'.}




(let loop ((i m))
   (begin
     (define i2 (* i i))
     (define ds (desc-series i2))
     ...))

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 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))


-------------------------------------------------------------------------
This SF.net email is sponsored by DB2 Express
Download DB2 Express C - the FREE version of DB2 express and take
control of your XML. No limits. Just data. Click to get it now.
http://sourceforge.net/powerbar/db2/