[Openmcl-devel] consing

John Miller millejoh at me.com
Tue May 14 14:10:09 UTC 2013


Ummm, tail recursion?

(dissasemble 'fac) might make for an interesting read. 

John

On May 14, 2013, at 5:37 AM, Taoufik Dachraoui <dachraoui.taoufik at gmail.com> wrote:

> Hi
> 
> This is probably simple but I could not figure out the explanation, I hope that
> someone could help me out
> 
> CL-USER> (defun fac (n) (if (= n 0) 1 (* n (fac (- n 1)))))
> FAC
> CL-USER> (time (fac 3))
> (FAC 3)
> took 0 milliseconds (0.000 seconds) to run.
> During that period, and with 1 available CPU core,
>      0 milliseconds (0.000 seconds) were spent in user mode
>      0 milliseconds (0.000 seconds) were spent in system mode
> 6
> 
> As you can see, there is no consing when FAC is called
> 
> Now, knowing that pushing an element on the stack, (PUSH 1 X) is equivalent
> to (SETQ X (CONS 1 X)), and since FAC is using a stack and thus pushing 
> values on the stack why there is no consing when (FAC 3) is called?
> 
> I believe there is a simple explanation but I could not figure it out
> 
> Kind regards
> Taoufik
> 
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