recursive function to find all the data nodes on which an operation node depends (up until some stopping nodes) - not required, but stashed here in case some of the ideas are

required_data nodes.R

# recursively find all the data nodes that this one depends on, stopping the
# search at any nodes that are data or whose unique names are in stop_at
required_data_nodes <- function (node, stop_at = c(), data_nodes = list()) {

  name <- node$unique_name

  if (!name %in% stop_at) {

    # if a data node, record and stop this search branch (data has no children)
    if (inherits(node, "data_node")) {

      data_nodes[[name]] <- node

    } else {

      # recurse through the children of this
      # node updating the data node list
      data_node_list <- lapply(node$children, required_data_nodes, stop_at, data_nodes)

      # combine and remove duplicates
      data_nodes <- do.call(c, data_node_list)
      data_nodes <- data_nodes[!duplicated(names(data_nodes))]

    }

  }

  data_nodes

}