[R] Writing a workable function

[Bill Venables]

Peter Dalgaard speculates:

> 
> Hmm. Perhaps that is a little too pessimistic... Sometimes one just
> need to provide a little information at the right spot to make things
> click into place.  In the present case the relevant point would seem
> to be that the result of an expression is not printed during function
> evaluations, 

Actually, no, that's not quite the root of the problem, although that
was a peripheral aspect of it.  If you look again at the example you
will see that the person was expecting that a local object generated
within a function would exist outside the function as a permanent
variable.  In other words that the function would behave just like a
macro in other languages.  This is the kind of misconception that is
classically very hard (in my limited experience) to turn around.
People actually *like* working with macros, strange as it may seem.

> but it would be nice also to have an example of why this
> is so - I'd go looking for an example of the kind
> 
> for (i in 1:1000) f(x) 
> 
> where f(x) is used for side effects but also has a visible value,
> which just isn't needed here. Such examples are not all that easy to
> come up with, but maybe it suffices to remind people that since every
> function has a value, one would have to invisible()ify it in every
> single user-written function to avoid having code like the above
> spewing out 1000 lines or more of garbage.

Yes, this is a point, but in this case the problem was more
fundamental.

> 
> > I'd say go ahead and have a go by all means, but be careful in what
> > you write as it is a very tricky and sensitive area.
> 
> Yes. The FAQ may not be the most obvious target, though. Recently,
> when someone got baffled by the behaviour of source(), it did cross my
> mind that maybe the section on that in the R-intro document could do
> with a couple of words on echo=TRUE. So perhaps R-intro would be a
> more natural place for this stuff too.

No real argument with any of these suggestions, but I do think the
problem is much deeper in this case.

Bill.
-- 
Bill Venables,      Statistician,     CMIS Environmetrics Project
CSIRO Marine Labs, PO Box 120, Cleveland, Qld,  AUSTRALIA.   4163
Tel: +61 7 3826 7251           Email: Bill.Venables at cmis.csiro.au    
Fax: +61 7 3826 7304      http://www.cmis.csiro.au/bill.venables/



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