promise: Create a new promise object

In promises: Abstractions for Promise-Based Asynchronous Programming

Create a new promise object

Description

promise() creates a new promise. A promise is a placeholder object for the eventual result (or error) of an asynchronous operation. This function is not generally needed to carry out asynchronous programming tasks; instead, it is intended to be used mostly by package authors who want to write asynchronous functions that return promises.

Usage

promise(action)

Arguments

action

A function with signature ⁠function(resolve, reject)⁠.

Details

The action function should be a piece of code that returns quickly, but initiates a potentially long-running, asynchronous task. If/when the task successfully completes, call resolve(value) where value is the result of the computation (like the return value). If the task fails, call reject(reason), where reason is either an error object, or a character string.

It's important that asynchronous tasks kicked off from action be coded very carefully–in particular, all errors must be caught and passed to reject(). Failure to do so will cause those errors to be lost, at best; and the caller of the asynchronous task will never receive a response (the asynchronous equivalent of a function call that never returns, i.e. hangs).

The return value of action will be ignored.

Value

A promise object (see then).

⁠action=⁠ formulas

With {promises} depending on R >= 4.1, the shorthand of a formula, ~ fn(.) for action is no longer recommended by the {promises} package or tidyverse (for example, {purrr}) as we now have access to the function shorthand, ⁠\(x) fn(x)⁠. Please update your action code to use the new function shorthand syntax ⁠\(resolve, reject) resolve(arg1, arg2)⁠ instead of ~ { resolve(arg1, arg2) }. The magically created resolve/reject functions can be confusing when chained with other methods.

Examples

# Create a promise that resolves to a random value after 2 secs
p1 <- promise(\(resolve, reject) {
  later::later(\() resolve(runif(1)), delay = 2)
})

p1 |> then(print)

# Create a promise that errors immediately
p2 <- promise(\(resolve, reject) {
  reject("An error has occurred")
})
then(p2,
  onFulfilled = \(value) message("Success"),
  onRejected = \(err) message("Failure")
)

promises documentation built on Nov. 5, 2025, 6:03 p.m.