R/chain.R

In itertools: Iterator Tools

#
# Copyright (c) 2009-2010, Stephen B. Weston
#
# This is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License as published
# by the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307
# USA

chain <- function(...) {
  iterators <- lapply(list(...), iter)

  nextEl <- function() {
    repeat {
      if (length(iterators) == 0) {
        stop('StopIteration', call.=FALSE)
      }

      tryCatch({
        return(nextElem(iterators[[1]]))
      },
      error=function(e) {
        if (identical(conditionMessage(e), 'StopIteration')) {
          iterators <<- iterators[-1]
        } else {
          stop(e)
        }
      })
    }
  }

  object <- list(nextElem=nextEl)
  class(object) <- c('abstractiter', 'iter')
  object
}