From: Mikkel Damsgaard <mikkel_damsgaard@...>
Date: 2001-09-22T02:23:59+09:00
Subject: [ruby-talk:21495] SV:  Re: SV:  Re: Ruby/objects book in style of The Little Lisper



> Lars Christensen wrote
> 
> On Fri, 21 Sep 2001, Mikkel Damsgaard wrote:
> 
> > Hey! Ruby is proper tail recursive, so since the recursive
> > factorial is much easier to understand and runs as fast, it
> > is a nice starting example. It would be cool to get back to it
> > later to explain tail recursiveness.
> 
> Is it?
> 
> The simple approach:
> 
> def fact(n)
>   if n == 1
>     1
>   else
>     n * fact(n-1)
>   end
> end
> 
> ... cannot be tail recusive because the multiplication is done after
> recursion. This version is a bit slower than the Kevin 
> Smith's imperative
> approach.
> 
> Rewriting the method so that it can be tail recursive:
> 
> def fact(n, m = 1)
>   return m if n == 1
>   fact(n-1, n * m)
> end
> 
> ... is even slower (more than 25% slower than the imperative 
> solution).
> 
> And finally, Ruby fails at recursive level five-thousand-something,
> reporting, "Stack level too deep" :-)
> 

Ups. I guess I was a little to trigger happy ;=)

Btw, why isn't it proper tail recursive? Matz?

/Mikkel