How to access a return value of a function that is being traced

Question

Is there any way of accessing return value of a function that is being traced by a function specified as exit param to trace? That sounds hard to understand, but I was not able to simplify the question without loosing the information. So here is a simple example.

We have a simple function

add10 <- function(a){
  a + 10
}

And some function that we want to be called when call to add10 exits.

trace.exit() <- function(){
...
}

Tracing is set up the following way.

trace(add10, exit=trace.exit)

And we do a call to add10

add10(5)

As I understand of right now, trace.exit will be called after add10 finished executing. Is there any way to access return value of add10 inside trace.exit?

I feel that there should be. But playing with sys.frames and looking through environments I was not able to get it.

The reason for doing so is a wish to capture all calls to some function and return values they give.

UPD Solution with wrapper or something similar is nice, but trace already implements a decorator pattern, so my question is about accessing return value from trace, not about solving the problem of decorators in R.

Answer 1

Why don't you use a wrapper that explicitly assigns the return value to a local variable:

add10 <- function(a){
  a + 10
}

wrap <- function(f) { function(...) { ..ret <- f(...) } }

add10_wrap <- wrap(add10)

trace.exit <- function() {
  cat(sprintf("Return value: %s\n", sys.frame(-1)$..ret))
}

trace(add10_wrap, exit=trace.exit)

add10_wrap(5)

One downside is that the wrapper will always return invisible results -- that's why the above example only prints the formatted output.