[plt-scheme] Re: HTDP: So you thought us empty? and empty

As much as I'd love to jump in and congratulate Guenther as well, I 
think its important to point out that the recursive call in the code he 
wrote is not tail-recursive.

For comparison purposes, here is some CL that I believe *is* 
tail-recursive.  Guenther: you should see if you can get the below 
working in your CL distribution (I don't have one handy to test it on). 
  If this version works without any need to increase the stack-size, 
then you really can't blame the folks at Xanalys (well, except that 
maybe they shouldn't impose a fixed limit on the stack size for their 
runtime).

(defun my-make-list (n)
    (my-make-list-accum n nil))

(defun my-make-list-accum (n accum)
   (cond
     ((< n 1) accum)
     (t (my-make-list (- n 1) (cons nil accum)))))

-Felix

>> (defun my-make-list (n)
>> (cond
>>  ((< n 1) nil)
>>  (t (cons nil (my-make-list (- n 1))))))


On Dec 6, 2003, at 8:13 PM, Matthias Felleisen wrote:

>  For list-related administrative tasks:
>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>
> Good for you. Good programmers stand up for their rights, and one of 
> them is tail-recursion. It's the only way to match your program's 
> structure to the structure of the data. Everything else is buggy over 
> the long run. -- Matthias
>
>
> On Saturday, December 6, 2003, at 08:02 PM, Guenther Schmidt wrote:
>
>>  For list-related administrative tasks:
>>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>>
>> Dear Matthias,
>>
>> apologies for *firing* this off.
>>
>> I'm switching back and forth between PLT Scheme and Lispworks (CL) 
>> and just to make it up to you I'll show you what I just *fired* at 
>> Xanalys five minutes before:
>>
>>
>> Hi,
>>
>> I heard that one of the differences between Scheme and CL is that 
>> Scheme is "proper tail recursive" and CL isn't. Until today I'm not 
>> 100% what that could mean but I think I'm seeing an effect of it 
>> here.
>>
>> (defun my-make-list (n)
>> (cond
>>  ((< n 1) nil)
>>  (t (cons nil (my-make-list (- n 1))))))
>>
>> If I run this on Lispworks with let's say merely n=200 I get a stack 
>> overflow, if I run it in Scheme with +100000 I just have to wait a 
>> little, but no stack overflow.
>>
>> What I really do like about CL and Scheme is the excessive use of 
>> recursion, and the parenthesis too.
>> Does this mean I need to switch back to iteration when dealing with 
>> lists of over 200 elements in CL?
>> Setting the *stack-size* to a higher level?
>>
>> If so ............. STUFF THAT!!!
>>
>> Guenther
>>
>>
>>
>> Matthias Felleisen wrote:
>>
>>>  For list-related administrative tasks:
>>>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>>> Use Pretty Big for now. -- Matthias
>>> On Saturday, December 6, 2003, at 07:20 PM, Guenther Schmidt wrote:
>>>>  For list-related administrative tasks:
>>>>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>>>>
>>>> ..... in HTDP.
>>>>
>>>> But as soon as one switches to MrED it turns out that's not even 
>>>> part of the standard.
>>>>
>>>> What exactly was the point of teaching us empty? and empty ?
>>>>
>>>> I mean I'm sure that I will find a function that is the equivalent 
>>>> of the above because there just has to be, but what was the point?
>>>>
>>>> Best regards
>>>>
>>>> Guenther
>>>>
>>>> BTW I figure it is "null"
>>>>
>>