exception - Is there a way to use tryCatch (or similar) in R as a loop, or to manipulate the expr in the warning argument?

Question

I have a regression model (lm or glm or lmer ...) and I do fitmodel <- lm(inputs) where inputs changes inside a loop (the formula and the data). Then, if the model function does not produce any warning I want to keep fitmodel, but if I get a warning I want to update the model and I want the warning not printed, so I do fitmodel <- lm(inputs) inside tryCatch. So, if it produces a warning, inside warning = function(w){f(fitmodel)}, f(fitmodel) would be something like

fitmodel <- update(fitmodel, something suitable to do on the model)

In fact, this assignation would be inside an if-else structure in such a way that depending on the warning if(w$message satisfies something) I would adapt the suitable to do on the model inside update.

The problem is that I get Error in ... object 'fitmodel' not found. If I use withCallingHandlers with invokeRestarts, it just finishes the computation of the model with the warning without update it. If I add again fitmodel <- lm(inputs) inside something suitable to do on the model, I get the warning printed; now I think I could try suppresswarnings(fitmodel <- lm(inputs)), but yet I think it is not an elegant solution, since I have to add 2 times the line fitmodel <- lm(inputs), making 2 times all the computation (inside expr and inside warning).

Summarising, what I would like but fails is:

tryCatch(expr = {fitmodel <- lm(inputs)},
         warning = function(w) {if (w$message satisfies something) {
                                    fitmodel <- update(fitmodel, something suitable to do on the model)
                                } else if (w$message satisfies something2){
                                    fitmodel <- update(fitmodel, something2 suitable to do on the model)

                                }
         }
)

What can I do?

The loop part of the question is because I thought it like follows (maybe is another question, but for the moment I leave it here): it can happen that after the update I get another warning, so I would do something like while(get a warning on update){update}; in some way, this update inside warning should be understood also as expr. Is something like this possible?

Thank you very much!

---

Generic version of the question with minimal example:

Let's say I have a tryCatch(expr = {result <- operations}, warning = function(w){f(...)} and if I get a warning in expr (produced in fact in operations) I want to do something with result, so I would do warning = function(w){f(result)}, but then I get Error in ... object 'result' not found.

A minimal example:

y <- "a"
tryCatch(expr = {x <- as.numeric(y)},
    warning = function(w) {print(x)})
Error in ... object 'x' not found

I tried using withCallingHandlers instead of tryCatch without success, and also using invokeRestart but it does the expression part, not what I want to do when I get a warning.

Could you help me?

Thank you!

Answer 1

It seems that you are looking for a functional wrapper that captures both the returned value and side effects of a function call. I think purrr::quietly is a perfect candidate for this kind of task. Consider something like this

quietly <- purrr::quietly

foo <- function(x) {
  if (x < 3)
    warning(x, " is less than 3")
  if (x < 4)
    warning(x, " is less than 4")
  x
}

update_foo <- function(x, y) {
  x <- x + y
  foo(x)
}

keep_doing <- function(inputs) {
  out <- quietly(foo)(inputs)
  repeat {
    if (length(out$warnings) < 1L)
      return(out$result)
    
    cat(paste0(out$warnings, collapse = ", "), "
")
    # This is for you to see the process. You can delete this line.
    
    if (grepl("less than 3", out$warnings[[1L]])) {
      out <- quietly(update_foo)(out$result, 1.5)
    } else if (grepl("less than 4", out$warnings[[1L]])) {
      out <- quietly(update_foo)(out$result, 1)
    }
  }
}

Output

> keep_doing(1)
1 is less than 3, 1 is less than 4 
2.5 is less than 3, 2.5 is less than 4 
[1] 4

> keep_doing(3)
3 is less than 4 
[1] 4