The prompt message will be displayed, and then readLines is used to
collect a single input value (possibly empty), which is then returned.
In most situations using the default con=stdin() should work
properly. Under RStudio, it is necessary to specify
con=file("stdin") for proper operation.
# use default promptask()
silly <- function() {
age <- ask("How old aroe you? ")
age <- as.numeric(age)
cat("In 10 years you will be", age + 10, "years old!\n")
}