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

i'm using lapply run complex function on large number of items, , i'd save output each item (if any) warnings/errors produced can tell item produced warning/error.

i found way catch warnings using withcallinghandlers (described here: https://stackoverflow.com/questions/4947528). however, need catch errors well. can wrapping in trycatch (as in code below), there better way 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 function is:

> catchtolist({warning("warning 1");warning("warning 2");1}) $value [1] 1  $warnings [1] "warning 1" "warning 2"  $error null  > catchtolist({warning("my warning");stop("my error")}) $value null  $warnings [1] "my warning"  $error [1] "my error" 

there several questions here on discuss trycatch , error handling, none found address particular issue. see how can check whether function call results in warning?, warnings() not work within function? how can 1 work around this?, , how tell lapply ignore error , process next thing in list? relevant ones.

maybe same solution, wrote factory convert plain old functions functions capture values, errors, , warnings, can

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

with each element of result containing value, error, , / or warnings. work user functions, system functions, or anonymous functions (factory(function(i) ...)). here's 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 helpers dealing 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)