Re: Tail recursion and hobbit (was: finite state machines in guile)

[Jim Blandy <jimb@red-bean.com>]

Yes, you're right --- I read the question as, "does Guile's
interpreter optimize tail calls as required by R4RS?" without looking
at the program text too carefully.

> > > Exactly. For instance, the following is not tail call optimized:
> > > 
> > > (define (fact n)
> > >   (if (<= n 1)
> > >       1
> > >       (* n fact (- n 1))))
> > > 
> > > even if equivalent to the original factorial. But guile's interpreter may
> > > also fail to optimize this (I have no way to verify this, any hint ?).
> > 
> > Guile's interpreter does do this correctly, by making the eval
> > function handle tail-position function application directly.  So this
> > function shouldn't allocate any new frames.
> 
> I am confused now. I guess the above example is to mean 
> (define (fact n)
>   (if (<= n 1)
>       1
>       (* n (fact (- n 1)))))
>            ^            ^ (Was that meant in the orig. post ?)
> 
> I thought that the recursive call to fact is _not_ in tail position
> (because the multiplication has to be carried out on the returned
> value). Or are you saying that guile's eval can do a CPS transform to put
> the call in tail position ??? That would be quite something ;)