R: show error and warning messages in foreach %dopar%

Question

I\'m new to using foreach() %dopar% for paralleling, and I have some problems about how it handles errors or warnings.

1. when I use try() with my customized e

Answer 1

I think the issue is around your errorhandling and a bit of a misunderstanding of dopar. First, think of dopar more like a function. It must return an object back to the user by default as a list (it gathers all the output from each worker and the foreach function gathers these and returns them to use, see 'output_list' below).

You can also set .errorhandling = 'pass', it will return the error back to the output object (note probably only works if .combine ='list'). Here is how .errorhandling works:

.errorhandling specifies how a task evaluation error should be handled. If the value is "stop", then execution will be stopped via the stop function if an error occurs. If the value is "remove", the result for that task will not be returned, or passed to the .combine function. If it is "pass", then the error object generated by task evaluation will be included with the rest of the results. It is assumed that the combine function (if specified) will be able to deal with the error object. The default value is "stop".

Here is an example of how to set up a tryCatch inside of a foreach. Note that errors are now stored in output_list.

output_list = foreach(i=1:3, .errorhandling='pass') %dopar% {
result <- tryCatch({
    object_that_doesnt_exist[i]}, 
     warning = function(war) {
         return('a warning')}, 
     error = function(err) {
         return('an error')}, 
     finally = {
         return('other things')
     }) # END tryCatch

  return(result)  # return your result to outputlist
}

output_list