Why does withCallingHandlers still stops execution?

Question

It seems that withCallingHandlers doesn\'t actually catch the error the way that tryCatch does and the script still stops executing.

Compar

Answer 1

Calling handlers provide a way of 'touching' a condition on the way through, maybe logging an error to a file before signalling the user in an interactive session.

Calling handlers can be used to 'muffle' warnings, messages, or errors if the calling handler doesn't actually return. You can make a calling handler not return using restarts -- surround the code that you'd like to continue executing in a call to withRestarts(), and invoke the restart in the handler:

f2 <- function() {
  cat("before tryCatch\n")
  withCallingHandlers({
      withRestarts({
          stop("this is an error!")
      }, muffleStop=function() {
          message("'stop' muffled")
      })
    },
    error = function(cond) {
      print(cond$message)
      invokeRestart("muffleStop")
    }
  )
  cat("after tryCatch\n")
}

It's more usual that the restarts are established in one chunk of code (like in the built-in function warning) and invoked in a completely independent chunk of code (like the built-in function suppressWarnings:

> warning
function (..., call. = TRUE, immediate. = FALSE, noBreaks. = FALSE, 
    domain = NULL) 
{
        ##
        ## ...
        ##
        withRestarts({
            .Internal(.signalCondition(cond, message, call))
            .Internal(.dfltWarn(message, call))
        }, muffleWarning = function() NULL)
        ##
        ## ...
        ##
}
<bytecode: 0x51a4730>
<environment: namespace:base>
> suppressWarnings
function (expr) 
{
    ops <- options(warn = -1)
    on.exit(options(ops))
    withCallingHandlers(expr, 
        warning = function(w) invokeRestart("muffleWarning"))
}
<bytecode: 0x35c2a60>
<environment: namespace:base>