Is there a way to get the warning parameter of a tryCatch to passthrough the result of the expr even if there is a handled warning?

r <- tryCatch(
  {
     warning("I'm a warning!")
     return(100) # I want the return value of the tryCatch to be this!
  },
  warning = function(w) {
    # send a notification with the warning message 
  }
)

The result of r should 100!

CodePudding user response：

tryCatch doesn’t do this: each handler interrupts the expression’s execution and immediately returns to the caller.

Instead, this is a task for withCallingHandlers, which handles a condition and then returns control back to the expression.

Here’s an example:

f = function () {
    warnings = list()
    self = environment()

    r = withCallingHandlers({
            warning("I'm a warning!")
            100
        },
        warning = function (w) {
            self$warnings = c(self$warnings, conditionMessage(w))
            invokeRestart('muffleWarning')
        }
    )

    list(r = r, warnings = warnings)
}

When we run it, this is the result:

f()
# $r
# [1] 100
#
# $warnings
# $warnings[[1]]
# [1] "I'm a warning!"

Note that you cannot use return() here. It’s usually redundant anyway but in this particular context it would abort the execution of f, so r would never get assigned a value, and the rest of the function wouldn’t be executed.

Incidentally, invokeRestart('muffleWarning') isn’t necessary; it suppresses the default handler which would print the warning to the standard error stream; you can remove it, and the call will work the same way, but the warning will additionally be printed. You can install your own restarts via withRestarts.

The condition system in R is incredibly powerful but also quite complex, and the documentation is unfortunately essentially nonexistent. The best resource I know is the Conditions chapter in Advanced R, as well as an essay adapted from Lisp, Beyond Exception Handling: Conditions and Restarts, which is (IMHO) less approachable but explains the concepts more fundamentally.