unpack_operator.R

# an 'unpack' operator, to do python/MatLab-like multiple assignment from
# functions

`%=%` <- function (expr, list) {
  
  if (!is.list(list))
    stop ('the right hand side must be a list')
  
  # grab the names to assign to
  expr <- substitute(expr)
  names_list <- lapply(expr, as.character)[-1]
  
  # drop any outputs for which the name was NULL
  blanks <- vapply(names_list, function (x) length(x) == 0, FALSE)
  if (any (blanks)) {
    list <- list[which(!blanks)]
    names_list <- names_list[which(!blanks)]
  }
  
  names <- unlist(names_list)
  
  # assign the names to the list
  names(list) <- names
  
  # do list2env into parent environment
  list2env(list, envir = parent.frame())
  
  invisible(NULL)
  
}

# ~~~~~~~~~~~~~~
# example

foo <- function() list(1, 2, 3)

# assign the elemtns to these names
{a; b; c} %=% foo()

a
# [1] 1
b
# [1] 2
c
# [1] 3

# skip (don't assign) one of the elements by using NULL instead of an object
{a1; NULL; c1} %=% foo()

a1
# [1] 1
c1
# [1] 3

# only these 5 objects were assigned
ls()
# [1] "%=%" "a"   "a1"  "b"   "c"   "c1"  "foo"