Advanced error handling

Question

I recently posed this question and thankfully was pointed to withRestarts() which seems pretty awesome and powerful to me :-) Now I'm eager to understand R's error handling capabilities in a bit more detail.

Actual questions

1. What is the recommended usage of simpleCondition()? Never used it before, but I thought it might be useful for designing custom errors and warnings that are in fact "true" conditions. Could it be used to build a database of specific conditions for which specific handlers are available?
2. Is there a way to "freeze" a certain state of the entire R workspace and return to it to restart a computation at a certain point? I'm aware of save.image(), but AFAIU, this doesn't store the "state" of the search path (search() or searchpaths()).

---

For those interested

Two code examples

1. illustration of my current use of withRestarts in dependence on this blog post
2. attempt to define a "custom condition"

I'd appreciate any comments/suggestion on what to do better ;-)

Example 1

require("forecast")
autoArimaFailsafe <- function(
    x,
    warning=function(w, ...) {
        message("autoArimaFailsafe> warning:")
        message(w)
        invokeRestart("donothing")},
    error=function(e, ...) {
        message("autoArimaFailsafe> error:")
        message(e)
        invokeRestart("abort")}
) {
    withRestarts(
        out <- tryCatch(
           {
                expr <- expression(auto.arima(x=x))
                return(eval(expr))
           },
           warning=warning,
           error=error 
        ),
        donothing=function(...) {
            return(eval(expr))
        },
        abort=function(...) {
            message("aborting")
            return(NULL)
        }
    )    
}
data(AirPassengers)
autoArimaFailsafe(x=AirPassengers)
autoArimaFailsafe(x="a")

Example 2

require("forecast")
autoArimaFailsafe <- function(
    x,
    warning=function(w, ...) {
        message("autoArimaFailsafe> warning")
        invokeRestart("donothing")},
    error=function(e, ...) {
        message("autoArimaFailsafe> error")
        invokeRestart("abort")},
    condition=function(cond, ...) {
        out <- NULL
        message(cond)
        condmsg     <- conditionMessage(c=cond)
        condclass   <- class(cond)
        if (any(class(cond) == "simpleWarning")) {
            out <- warning(w=cond)
        } else if (any(class(cond) == "simpleError")) {
            out <- error(e=cond)
        } else if (any(class(cond) == "simpleCondition")) {
            if (condmsg == "invalid class: character") {
                out <- invokeRestart("forcedefault")
            }
        }
        return(out)
    }
) {
    withRestarts(
        out <- tryCatch(
           {
                expr <- expression(auto.arima(x=x))
                if (class(x) == "character") {
                    expr <- signalCondition(
                        simpleCondition("invalid class: character", 
                            call=as.call(expr))
                    )
                }
                return(eval(expr))
           },
           condition=condition
        ),
        donothing=function(...) {return(eval(expr))},
        abort=function(...) {
            message("aborting")
            return(NULL)
        },
        forcedefault=function(...) {
            data(AirPassengers)
            expr <- expression(auto.arima(x=AirPassengers))
            return(eval(expr))
        } 
    )
}
autoArimaFailsafe(x=AirPassengers)
autoArimaFailsafe(x=NULL)
autoArimaFailsafe(x="a")

Answer 1

This post references the inspiration for R's condition handling.

For 1., I think of simpleCondition as illustrating how one can construct custom conditions, e.g,.

 myCondition <-
    function(message, call=NULL, type=c("overflow", "underflow", "zero"))
{
    type <- match.arg(type)             # only allowed types past here
    class <- c(type, "my", "condition")
    structure(list(message = as.character(message), call = call), 
        class = class)
}

is a constructor for making custom conditions

> myCondition("oops")
<overflow: oops>
> myCondition("oops", type="underflow")
<underflow: oops>

These conditions can be used in tryCatch or withCallingHandlers

xx <- tryCatch({
    signalCondition(myCondition("oops", type="underflow"))
}, underflow=function(e) {
    message("underflow: ", conditionMessage(e))
    NA # return value, assigned to xx
})

These are S3 classes so can have a linear hierarchy -- bad and worse are both subclasses of error.

myError <-
    function(message, call=NULL, type=c("bad", "worse"))
{
    type <- match.arg(type)
    class <- c(type, "error", "condition")
    structure(list(message=as.character(message), call=call),
              class=class)
}

One might also create an error that extends the 'simpleError' S3 class as cond <- simpleError("oops"); class(cond) = c("myerr", class(cond)

With tryCatch we just get access to a single handler, the first (in the sense described on ?tryCatch) to match the class of condition

tryCatch({
    stop(myError("oops", type="worse"))
}, bad = function(e) {
    message("bad error: ", conditionMessage(e))
}, worse = function(e) {
    message("worse error: ", conditionMessage(e))  # here's where we end up
}, error=function(e) {
    message("error: ", conditionMessage(e))
})

With withCallingHandlers we have the opportunity to hit multiple handlers, provided we don't invoke a restart

withCallingHandlers({
    stop(myError("oops", type="bad"))
}, bad = function(e) {                             # here...
    message("bad error: ", conditionMessage(e))
}, worse = function(e) {
    message("worse error: ", conditionMessage(e))
}, error=function(e) {                             # ...and here...
    message("error: ", conditionMessage(e))
})                                                 # ...and top-level 'error'

withCallingHandlers({
    x <- 1
    warning(myError("oops", type="bad"))
    "OK"
}, bad = function(e) {                     # here, but continue at the restart
    message("bad warning: ", conditionMessage(e))
    invokeRestart("muffleWarning")
}, worse = function(e) {
    message("worse warning: ", conditionMessage(e))
})

I'm not so sure about your question 2; I think this is the situation that calling handlers are designed to address -- the entire frame where the condition was invoked is poised waiting to continue, once you invoke the restart.