setClass not found when running R script from command line

I have simple R script which trying to Define class. example.R

 Tuple <- setClass("Tuple",
            slots = c(
              id="character",
              comp="character",
              stream="character",
              task="character",
              output="vector",
              anchors="vector"
                )
            );

when i run Rscript example.R . i am getting an error saying below

Error in eval(expr, envir, enclos) : could not find function "setClass" Calls: source -> withVisible -> eval -> eval

can any one help?