Cache the result of an expression in R

cache.R

#' Cache the result of an expression.
#'
#' Use \code{options(cache.path = "...")} to change the cache directory (which
#' is the current working directory by default).
#'
#' @param expr expression to evaluate
#' @param key basename for cache file
#' @param ignore_cache evalute expression regardless of cache file?
#' @return result of expression or read from cache file
#'
#' @example
#' answer <- cache({
#'   \dontrun{
#'   Sys.sleep(7500000 * 365 * 86400)
#'   }
#'   42
#' }, "life_universe_everything")
#'
#' @seealso \code{\link[R.cache]}
cache <- function(expr, key, ignore_cache = FALSE) {
  filename <- file.path(getOption("cache.path", "."), paste0(key, ".rds"))
  if (!ignore_cache && file.exists(filename)) {
    message(sprintf("Loading result from %s", filename))
    result <- readRDS(filename)
  } else {
    result <- expr
    message(sprintf("Saving result to %s", filename))
    saveRDS(result, filename)
  }
  result
}

Thanks @jamesonquinn, that makes sense. This allows you to refresh the cache in an interactive way, rather than changing the code.

You could even invalidate the cache automatically when the parse tree of the cached expression changes.

`%<cache-%` <- function(key, value) {
  key <- substitute(key)
  stopifnot(is.name(key))
  previoushash = "none"
  hash <- digest::digest(substitute(value))

  filename <- file.path(getOption("cache.path", "."), paste0(deparse(key), ".rds"))
  hashfilename <- file.path(getOption("cache.path", "."), paste0(deparse(key), ".hash"))
  if (file.exists(hashfilename)) {
    previoushash <- readRDS(hashfilename)
  }
  if (file.exists(filename) & (hash == previoushash) & !getOption("refresh.cache", F)) {
    message(sprintf("Loading result from %s", filename))
    value <- readRDS(filename)
  } else {
    message(sprintf("Saving result to %s", filename))
    saveRDS(value, filename)
    saveRDS(hash, hashfilename)
  }

  assign(as.character(key), value, env = parent.frame())
  invisible(key)
}