[R] Evaluation of defaults in functions

Prof Brian Ripley ripley at stats.ox.ac.uk
Fri Sep 29 08:03:46 CEST 2006

On Thu, 28 Sep 2006, hadley wickham wrote:

>> And, to answer the specific question: Yes, R has lazy evaluation,
>> everywhere. Arguments are always evaluated if and when they are
>> needed.
> But doesn't R has a rather limited force of lazy evaluation? - you
> have no control over it, apart from that arguments are evaluated
> lazily.  This rather limited compared to other languages (no lazy
> lists etc)

I'd say that was a rather pure form.

However, Peter's statement is not quite true. R has several types of 
functions, and I think he is referring to closures, the functions written 
in R itself.  The argument handling in the built-in functions (primitive 
and .Internal) is different, and they will often evaluate all the 
arguments whether needed or not.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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