How to catch both warnings and errors?

Dear expeRts,

I am struggling with warning/error handling.

I would like to call a function which can produce either
a) normal output
b) a warning
c) an error

Since the function is called several (thousand) times in a loop, I  
would like
to proceed "quietly" and collect the warnings and errors [to deal  
with them at a
later point].
I do not see the function warnings() being used below:

?warnings

It delivers the stored warnings with different default behavior for  
interactive and non-interactive sessions.
I have seen constructs with tryCatch (which can deal with errors)  
and with
withCallingHandlers (which can deal with warnings), but I cannot  
figure out how
to catch *both* warnings and errors. Below is a minimal example of  
the function
that is called several times in a large loop. The function should  
catch warnings
and errors; the former work fine, but with the latter I do not know  
how to proceed.

Made some changes in you code but don't know if it is what you were  
hoping for:

f <- function(x){
    ## warnings
    w.list <- NULL # init warning
    w.handler <- function(w){ # warning handler
	    warn <- simpleWarning(w$message, w$call) # build warning
# first change here
        w.list <<- c(w.list, paste(warnings(), collapse = " ")) # save  
warning
        invokeRestart("muffleWarning")
        }
    ## errors
      e.list <- NULL # init errors   # not sure this is  good idea
      e.handler <- function(e){ # error handler
	     err <- c(e.list, e$message, e$call) # save error
	     return( err)
         }
    ## execute command
# wrapped cal in try()
    res <- withCallingHandlers(try(log(x)), warning = w.handler, error  
= e.handler)
    ## return result with warnings and errors
    list(result = res, warning = w.list, error = e.list)
}
The function should *always* return the list with the three  
components.

How can I achieve this?

Cheers,

Marius

Ps: How can I get the warning/error message in a nice string (as it  
is printed
in a normal warning/error message)? The approach shown below is  
somehow ugly.
Basically, I had trouble converting the call w$call to a string.

## based on http://tolstoy.newcastle.edu.au/R/help/04/06/0217.html
## and http://www.mail-archive.com/r-help at r-project.org/msg81380.html
f <- function(x){
   ## warnings
   w.list <- NULL # init warning
   w.handler <- function(w){ # warning handler
	warn <- simpleWarning(w$message, w$call) # build warning
       w.list <<- c(w.list, paste(warn, collapse = " ")) # save  
warning
       invokeRestart("muffleWarning")
   }
   ## errors
   e.list <- NULL # init errors
   e.handler <- function(e){ # error handler
	err <- simpleError(e$message, e$call)
	e.list <<- c(e.list, paste(err, collapse = " ")) # save error
   }
   ## execute command
   res <- withCallingHandlers(log(x), warning = w.handler, error =  
e.handler)
   ## return result with warnings and errors
   list(result = res, warning = w.list, error = e.list)
}

f(1)
f(-1)
f("a")

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How to catch both warnings and errors?

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