recursive list apply for R

rsapply.R

#' Recursive list apply, given a bunch of vectors and a function, creates lists of lists with the elements as keys
#'  and values determined by fn.
#' @param ... vectors to loop over
#' @param fn function to execute for each leaf
#' @examples
#' grip:::rsapply(c('a', 'b'), c('f', 'g'), fn=function(x,y) c(x, y))
#' #> list(a=list(f=c('a', 'f'), g=c('a', 'g')), b=list(f=c('b', 'f'), g=c('b', 'g')))
rsapply <- function(..., fn) {
  .rsapply <- function(el=NULL, ..., args=c(), fn) {
    if(is.null(el)) return(do.call(fn, as.list(args)))
    result <- list()
    for(x in el) {
      result[[x]] <- rsapply(..., args=c(args, x), fn=fn)
    }
    result
  }
  .rsapply(..., fn=fn)
}