[R] static analysis tools for R code?

[Vivek Rao]

An R script will terminate when one tries to use an
undefined variable, with a message such as 

Error in print(x) : Object "x" not found

This run-time error might occur after the script has
already been running for some time. In some cases it
would be nice to get such warnings before the script
is run, just as a syntax error caused by a missing
parenthesis is caught.

Are there any "static analysis" tools for R? Such a
tool would not have to be perfect to be useful.
Besides using undefined variables, defining variables
that are never used is something I'd like to be warned
about.