A minimal working example.
See script.R for the R code.
It checks whether the snakemake rule includes a log directive by checking
the length of the log slot of the snakemake S4 object.
If the log file path is given, it writes all messages, warnings, etc to the log file.
Otherwise, this output is printed to the console like usual.
The Snakefile includes an example rule with a log and without a log file.
Both run without errors.