Experimental lifecycle


  expr = NULL,
  envir = parent.frame(),
  substitute = TRUE,
  globals = TRUE,
  packages = NULL,
  queue = future_promise_queue()



An R expression. While the expr is eventually sent to future::future(), please use the same precautions that you would use with regular promises::promise() expressions. future_promise() may have to hold the expr in a promise() while waiting for a future worker to become available.


The environment from where global objects should be identified.


If TRUE, argument expr is substitute():ed, otherwise not.


(optional) a logical, a character vector, or a named list to control how globals are handled. For details, see section 'Globals used by future expressions' in the help for future().


(optional) a character vector specifying packages to be attached in the R environment evaluating the future.


extra parameters provided to future::future()


A queue that is used to schedule work to be done using future::future(). This queue defaults to future_promise_queue() and requires that method queue$schedule_work(fn) exist. This method should take in a function that will execute the promised future work.


Unlike [future::future()], future_promise()returns a [promise()] object that will eventually resolve the \pkg{future}expr`.


When submitting future work, future (by design) will block the main R session until a worker becomes available. This occurs when there is more submitted future work than there are available future workers. To counter this situation, we can create a promise to execute work using future (using future_promise()) and only begin the work if a future worker is available.

Using future_promise() is recommended when ever a continuous runtime is used, such as with plumber or shiny.

For more details and examples, please see the vignette("future_promise", "promises") vignette().


  • future_promise_queue: Default future_promise() work queue to use. This function returns a WorkQueue that is cached per R session.

  • future_promise: Creates a promise() that will execute the expr using future::future().

See also


# Relative start time start <- Sys.time() # Helper to force two `future` workers with_two_workers <- function(expr) { if (!require("future")) { message("`future` not installed") return() } old_plan <- future::plan(future::multisession(workers = 2)) on.exit({future::plan(old_plan)}, add = TRUE) start <<- Sys.time() force(expr) while(!later::loop_empty()) {Sys.sleep(0.1); later::run_now()} invisible() } # Print a status message. Ex: `"PID: XXX; 2.5s promise done"` print_msg <- function(pid, msg) { message( "PID: ", pid, "; ", round(difftime(Sys.time(), start, units = "secs"), digits = 1), "s " , msg ) } # `"promise done"` will appear after four workers are done and the main R session is not blocked # The important thing to note is the first four times will be roughly the same with_two_workers({ promise_resolve(Sys.getpid()) %...>% print_msg("promise done") for (i in 1:6) future::future({Sys.sleep(1); Sys.getpid()}) %...>% print_msg("future done") })
#> Loading required package: future
{ #> PID: XXX; 2.5s promise done #> PID: YYY; 2.6s future done #> PID: ZZZ; 2.6s future done #> PID: YYY; 2.6s future done #> PID: ZZZ; 2.6s future done #> PID: YYY; 3.4s future done #> PID: ZZZ; 3.6s future done }
# `"promise done"` will almost immediately, before any workers have completed # The first two `"future done"` comments appear earlier the example above with_two_workers({ promise_resolve(Sys.getpid()) %...>% print_msg("promise") for (i in 1:6) future_promise({Sys.sleep(1); Sys.getpid()}) %...>% print_msg("future done") }) { #> PID: XXX; 0.2s promise done #> PID: YYY; 1.3s future done #> PID: ZZZ; 1.4s future done #> PID: YYY; 2.5s future done #> PID: ZZZ; 2.6s future done #> PID: YYY; 3.4s future done #> PID: ZZZ; 3.6s future done }