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Re: Writing a procedure in different style

From: Zelphir Kaltstahl
Subject: Re: Writing a procedure in different style
Date: Sun, 20 Dec 2020 18:57:34 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Icedove/68.10.0

Hello Tomas!

I think you are right about it only being down one stack frame down. The
checks are performed on what contains the next thing which is recurred on.

For a moment I thought "But isn't the null? check done twice in the
first cond part?" But then I realized, that the rest is split up into
car and cdr before recurring on it and that the first check if for the
whole of rest, while in the next iteration, the check would only be for
the car of rest. So no duplicated work.

On 13.12.20 16:43, wrote:
> On Sun, Dec 13, 2020 at 04:01:24PM +0100, Zelphir Kaltstahl wrote:
>> Hi Tomas!
>> In some way what you write makes sense. Let me state here, that I did
>> read that book and worked through it for a year though, even through the
>> complicated parts like the y-combinator and some chapters I must have
>> read like 4 or 5 times and discovered new aspects on each try.
> That's it -- I'm through some n-th iteration and still go "oh!" from
> time to time :-D
>> What is typically the case in the book is a different situation though,
>> than what was in Taylan's procedure. Usually it is the list you are
>> working on in that iteration, which you check for being (null? ...), not
>> the thing, that you give as argument to a recursive call or as a return
>> value, which you add in some way to the result. Usually the questions
>> from the quote are asked once the argument is received in the next
>> iteration. That I definitely usually do, but in Taylan's answer there is
>> an (if (null? ...) ...) for the `rest`, inside the case, where the usual
>> (null? ...) check is already done on the subtree, which we recur on.
> I have the hunch that this is only shifting things one level
> up or down the stack, but basically, it's the same principle
> at work. I'd have to fiddle for a while with that to see whether
> I'm totally off, though.
> Anyway, a reminder for me to do the n+1st iteration: "Do It, Do
> It Again, and Again, and Again, ..." :-)
> Thanks for that!
> Cheers
>  - t



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