disabling of mc.preschedule in mcapply not possible with plyr

01-mcapply.R

# file: <multicore>/R/mcapply.R
mclapply <- function(X, FUN, ..., mc.preschedule=TRUE, mc.set.seed=TRUE, mc.silent=FALSE, mc.cores=getOption("cores"), mc.cleanup=TRUE) {
  # skip code...
  if (!mc.preschedule) { # sequential (non-scheduled)
    # => reaching this code is my goal!
    # skip code...
  }
  # skip code...
}

02-doMC.R

# file: <doMC>/R/doMC.R
doMC <- function(obj, expr, envir, data) {
  # set the default mclapply options
  preschedule <- TRUE
  set.seed <- TRUE
  silent <- FALSE
  cores <- workers(data)
  # skip code...
  # check for multicore-specific options
  options <- obj$options$multicore
  if (!is.null(options)) {
    nms <- names(options)
    recog <- nms %in% c('preschedule', 'set.seed', 'silent', 'cores')
    if (any(!recog))
      warning(sprintf('ignoring unrecognized multicore option(s): %s',
                      paste(nms[!recog], collapse=', ')), call.=FALSE)

    if (!is.null(options$preschedule)) {
      if (!is.logical(options$preschedule) || length(options$preschedule) != 1) {
        warning('preschedule must be logical value', call.=FALSE)
      } else {
        if (obj$verbose)
          cat(sprintf('setting mc.preschedule option to %d\n', options$preschedule))
        preschedule <- options$preschedule
      }
    }
    # skip code...
  }
  # skip code...
}

03-foreach.R

# file: <foreach>/R/foreach.R
foreach <- function(..., .combine, .init, .final=NULL, .inorder=TRUE,
                    .multicombine=FALSE,
                    .maxcombine=if (.multicombine) 100 else 2,
                    .errorhandling=c('stop', 'remove', 'pass'),
                    .packages=NULL, .export=NULL, .noexport=NULL,
                    .verbose=FALSE) {
  # skip code...
  args <- substitute(list(...))[-1]

  if (length(args) == 0)
    stop('no iteration arguments specified')
  argnames <- names(args)
  if (is.null(argnames))
    argnames <- rep('', length(args))

  # check for backend-specific options
  options <- list()
  opts <- grep('^\\.options\\.[A-Za-z][A-Za-z]*$', argnames)
  if (length(opts) > 0) {
    # put the specified options objects into the options list
    for (i in opts) {
      bname <- substr(argnames[i], 10, 100)
      options[[bname]] <- list(...)[[i]]
    }

    # remove the specified options objects from args and argnames
    args <- args[-opts]
    argnames <- argnames[-opts]
  }
  # skip code...
}

04-ply-list.r

# file: <plyr>/R/ply-list.r
llply <- function(.data, .fun = NULL, ..., .progress = "none", .inform = FALSE, .parallel = FALSE) {
  # skip code...
  if (.parallel) {
    # skip code...
    result <- foreach(i = seq_len(n)) %dopar% do.ply(i) # => no possibility to add optional code to foreach() call!
  } else {
    result <- loop_apply(n, do.ply)
  }
  # skip code...
}