How do I save warnings and errors as output from a function?

Question

I'm using `lapply` to run a complex function on a large number of items, and I'd like to save the output from each item (if any) together with any warnings/errors that were produced so that I can tell which item produced which warning/error. I found a way to catch warnings using `withCallingHandlers` (described here). However, I need to catch errors as well. I can do it by wrapping it in a `tryCatch` (as in the code below), but is there a better way to do it? ``` catchToList <- function(expr) { val <- NULL myWarnings <- NULL wHandler <- function(w) { myWarnings <<- c(myWarnings, w$message) invokeRestart("muffleWarning") } myError <- NULL eHandler <- function(e) { myError <<- e$message NULL } val <- tryCatch(withCallingHandlers(expr, warning = wHandler), error = eHandler) list(value = val, warnings = myWarnings, error=myError) } ``` Sample output of this function is: ``` > catchToList({warning("warning 1");warning("warning 2");1}) $value [1] 1 $warnings [1] "warning 1" "warning 2" $err…

Accepted Answer

Maybe this is the same as your solution, but I wrote a `factory` to convert plain old functions into functions that capture their values, errors, and warnings, so I can

```
test <- function(i)
    switch(i, "1"=stop("oops"), "2"={ warning("hmm"); i }, i)
res <- lapply(1:3, factory(test))
```

with each element of the result containing the value, error, and / or warnings. This would work with user functions, system functions, or anonymous functions (`factory(function(i) ...)`). Here's the factory

```
factory <- function(fun)
    function(...) {
        warn <- err <- NULL
        res <- withCallingHandlers(
            tryCatch(fun(...), error=function(e) {
                err <<- conditionMessage(e)
                NULL
            }), warning=function(w) {
                warn <<- append(warn, conditionMessage(w))
                invokeRestart("muffleWarning")
            })
        list(res, warn=warn, err=err)
    }
```

and some helpers for dealing with the result list

```
.has <- function(x, what)
    !sapply(lapply(x, "[[", what), is.null)
hasWarning <- function(x) .has(x, "warn")
hasError <- function(x) .has(x, "err")
isClean <- function(x) !(hasError(x) | hasWarning(x))
value <- function(x) sapply(x, "[[", 1)
cleanv <- function(x) sapply(x[isClean(x)], "[[", 1)
```

How do I save warnings and errors as output from a function?

Solution

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