From luke-tierney at uiowa.edu  Mon Apr 18 16:41:58 2011
From: luke-tierney at uiowa.edu (luke-tierney at uiowa.edu)
Date: Mon, 18 Apr 2011 09:41:58 -0500
Subject: [Rd] Tail Call Elimination?
In-Reply-To: <BANLkTim3Y+zfNBBjGTXE4PijRqv41Cs-nw@mail.gmail.com>
References: <BANLkTim3Y+zfNBBjGTXE4PijRqv41Cs-nw@mail.gmail.com>
Message-ID: <alpine.DEB.2.00.1104180941470.1955@luke-inspiron>

The premise of your post is false: contrary to popular belief, R's
looping constructs are not particularly inefficient. Slowness of loops
relative to vectorized code comes from the cost of interpreting the
body of the loop.  That exact same interpreter would be used to
interpret the bodies of functions used to express loops using
recursion, so it is not reasonable to expect this to improve
performance. In fact, due to the inefficiency of the current function
call mechanism in R, quite the opposite is true.

As to the question: tail call optimization cannot be applied in R, at
least not in a simple way, because the semantics of R provide access
to the call stack via the sys.xyz functions and parent.frame and such.
It might be possible to make some semantic changes, such as only
guaranteeing access to the immediate caller, but there isn't much
point unless/until the performance of the function calling mechanism is
improved.

Best,

luke

On Sun, 17 Apr 2011, Mohit Dayal wrote:

> Dear R-programmers,
>
> I am trying to program a Newton-Raphson in R (yes, i will try GSL, not right
> now), and it would be a real boon if R had tail call elimination, so that a
> recursive program has a guarantee not to fail due to stack overflows, given
> how slow loops in R are. I did look at the documentation, but could not find
> a reason for it.
>
> Regards,
> Mohit Dayal
> Researcher
> Applied Statistics & Computing Lab
> ISB
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
Luke Tierney
Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:      luke at stat.uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu



[Rd] Tail Call Elimination?
luke-tierney at uiowa.edu