Deliver a message after returning the function result

Question

Consider the following function foo

foo <- function(x) {
    print(x)
    message("test message")
}

I'd like to deliver the message after the result of the function so that if the result is long I don't have to scroll up to see if there was an important message (or change my max.print option). The issue is when I want to assign the result without printing the actual result.

Is there a way to print the result of the function, followed by the message, but also so that when I assign the result nothing at all is printed? Ideally, I'd like to avoid the use of print altogether.

The desired result without assignment is

> foo(5)                            
# [1] 5
# test message 

The desired result with assignment is

> bar <- suppressMessages(foo(5))
> bar
# [1] 5

Answer 1

You can achieve this by creating a class for your foo function, e.g. bar, and then creating a print method for this new class.

For example:

foo <- function(x) {
  class(x) <- c("bar", class(x))
  x
}


print.bar <- function(x, message=TRUE, ...){
  class(x) <- setdiff("bar", class(x))
  NextMethod(x)
  if(message)   message("test message")

}

Now try it:

foo(5)
[1] 5
test message

With assignment:

x <- foo(5)
x
[1] 5
test message

Some other ways of interacting with the print method:

print(x, message=FALSE)
[1] 5

suppressMessages(print(x))
[1] 5