[Rd] what is the preferred method to create a package local variable?

[Martin Morgan]

Hi Whit --

Whit Armstrong <armstrong.whit at gmail.com> writes:

> for the moment, I'm using:
>
> .onAttach <- function(libname, pkgname) {
>     .bbg.db.conn <<- dbConnect(dbDriver("PostgreSQL"), user="blah","blah")
> }
>
> .onUnload <- function(libpath) {
>     dbDisconnect(.bbg.db.conn)
> }
>
>
> which results in a hidden global variable in the global environment.
>
> I would prefer to make the assignment only in the package namespace.
> I've looked at assignInNamespace, but I can't seem to make it work.
>
> Is there a preferred method for doing this?

I don't konw about preferred, but one method is

pkgVars <- local({
    x <- NULL
    list(getX=function() x, setX=function(value) x <<- value)
})

with use

> pkgVars$getX()
NULL
> pkgVars$setX(123)
> pkgVars$getX()
[1] 123

This introduces a different programming paradigm (a list of functions)
that might not be familiar for end-users and is not readily amenable
to documentation. A probably better way is

pkgVars <- new.env(parent=emptyenv())
getX <- function() get("x", pkgVars, inherits=FALSE)
setX <- function(value) assign("x", value, envir=pkgVars)

> getX()
Error in get("x", pkgVars, inherits = FALSE) : object 'x' not found
> setX(123)
> getX()
[1] 123

A tidier response to the first getX() could be arranged with something
like

pkgVars <- local({
    env <- new.env(parent=emptyenv())
    env[["x"]] <- NULL
    env
})

pkgVars, getX, and setX can be exported from the name space or not.

FWIW, usually one wants .onLoad rather than .onAttach, so that a
package import-ing your package (not just depend-ing, or when used
directly in an interactive session) will execute the setup code.

Hope that helps and is not too misleading.

Martin

> When I try adding an assignment directly in the source file, I get the
> "cannot change value of locked binding" error.
>
> What am I missing?
>
> Thanks,
> Whit
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

-- 
Martin Morgan
Computational Biology / Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N.
PO Box 19024 Seattle, WA 98109

Location: Arnold Building M2 B169
Phone: (206) 667-2793