[Rd] Loose code in R package files

[Prof Brian Ripley]

On Tue, 14 Jun 2011, Stephen Ellison wrote:

> I'm sure i've seen the answer to this, but can't find it:
>
> If there is executable code in an R package .R file that does not 
> return a function (that is, something like x <- rnorm(5), outside 
> any function body ), what will actually happens at build and load 
> time?
>
> And (more importantly, so I can point a colleague to documented 
> guidance on the matter) is there somewhere in R's docs that says why 
> this is not likely to be a good idea and suggests the sort of things 
> it is sensible - or not sensible - to include in .R files?

It is documented in 'Writing R Extensions':

The R subdirectory contains R code files, only.
...
It should be possible to read in the files using source(), so R 
objects must be created by assignments. Note that there need be no 
connection between the name of the file and the R objects created by 
it. Ideally, the R code files should only directly assign R objects 
and definitely should not call functions with side effects such as 
require and options. If computations are required to create objects 
these can use code `earlier' in the package (see the `Collate' field) 
plus, only if lazyloading is used, functions in the `Depends' packages 
provided that the objects created do not depend on those packages 
except via name space imports. (Packages without name spaces will work 
under somewhat less restrictive assumptions.)

So your example creates an object 'x' in the package or name space. 
Which is perfectly legal, but maybe not intentional.  For example, R's 
base package does

## needs to run after paste()
.leap.seconds <- local({
     .leap.seconds <-
         c("1972-6-30", "1972-12-31", "1973-12-31", "1974-12-31",
           "1975-12-31", "1976-12-31", "1977-12-31", "1978-12-31",
           "1979-12-31", "1981-6-30", "1982-6-30", "1983-6-30",
           "1985-6-30", "1987-12-31", "1989-12-31", "1990-12-31",
           "1992-6-30", "1993-6-30", "1994-6-30","1995-12-31",
           "1997-6-30", "1998-12-31", "2005-12-31", "2008-12-31")
     .leap.seconds <- strptime(paste(.leap.seconds , "23:59:60"),
                               "%Y-%m-%d %H:%M:%S")
     c(as.POSIXct(.leap.seconds, "GMT")) # lose the timezone
})


S Ellison
-- 
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)
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