Re: Newbie problem: naked left-hand reference

On Thu, 19 Apr 2007, Victor Rodriguez wrote:

> 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.
>

>From r5rs 4.2.2:

"One restriction on letrec is very important: it must be
possible to evaluate each hiniti without assigning or referring
to the value of any hvariablei. If this restriction is
violated, then it is an error."



> 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,
>

No, I don't agree thats better style. In my opinion, the original code 
should have been valid. It will probably be valid in r6rs though.


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