print if not assigned

Question

How can I write into a function a way to detect if the output is being assigned (<-) to something? The reasoning is I\'d like to print a message if it is no

Answer 1

I think the best you can do is to define a special print method for objects returned by the function:

## Have your function prepend "myClass" to the class of the objects it returns
fun <- function(x) {
    class(x) <- c("myClass", class(x))
    x
}

## Define a print method for "myClass". It will be dispatched to 
## by the last step of the command line parse-eval-print cycle.
print.myClass <- function(obj) {
    cat("message\n")
    NextMethod(obj)
}

> fun(1:10)
message
 [1]  1  2  3  4  5  6  7  8  9 10
attr(,"class")
[1] "myClass"
>
> out <- fun(1:10)
>