[R-pkg-devel] if statements in NAMESPACE file

[Duncan Murdoch]

On 29/09/2021 2:13 p.m., Andrew Simmons wrote:
> I noticed in the Writing R Extensions manual, it says that within a
> NAMESPACE file, "Only very simple conditional processing of if statements
> is implemented.".
> I tried it out myself by importing a Windows exclusive function:
> 
> 
> if (.Platform$OS.type == "windows")
>      importFrom(utils, getWindowsHandles)
> 
> 
> It also says in the manual that a NAMESPACE file "is not processed as R
> code", so I'm wondering if anyone knows exactly how if statements are
> handled within a NAMESPACE file.

For a question like this, going to the source is really the only choice. 
  NAMESPACE is parsed when the package is installed, using the function 
parseNamespaceFile from the base package.

What that function does is to parse NAMESPACE as if it is R code, then 
go through the unevaluate expressions.  If it comes across one that is 
an "if", it evaluates it in the global environment.  But usually that's 
not *your* global environment, that's the global environment that is in 
place while the package is being installed.

If the condition is TRUE, it handles the clause that follows; if FALSE, 
it will look at the "else" clause.

So your

   if (.Platform$OS.type == "windows")

would be fine because .Platform is in the base package, so it's 
guaranteed to be available.  I suspect something like

   if (stats::runif(1) > 0.5) export(someFunction)

would work too, for a particularly frustrating experience for your 
users.  It would mean half the installs export the function, and half don't.

Duncan Murdoch