no-assign.R

`:=` <- function(lhs, rhs) {
  var <- deparse(substitute(lhs))
  if (exists(var, parent.frame(), inherits = FALSE)) {
    stop("Variable `", var, "` is already defined.", call. = FALSE)
  } else {
    assign(var, rhs, parent.frame())
  }
}

# An alternative version making use of `lockBinding`. This prevents users from overwriting
# the variable using `<-`, `->` or `=` later on. The former variant would only throw an error
# when using `:=` again.
`:=` <- function(var, value) {
  id <- deparse(substitute(var))
  assign(id, value, parent.frame())
  lockBinding(id, parent.frame())
  invisible(value)
}