promise_map: Promise-aware lapply/map

In promises: Abstractions for Promise-Based Asynchronous Programming

Promise-aware lapply/map

Description

Similar to base::lapply() or purrr::map, but promise-aware: the .f function is permitted to return promises, and while lapply returns a list, promise_map returns a promise that resolves to a similar list (of resolved values only, no promises).

Usage

promise_map(.x, .f, ...)

Arguments

.x	A vector (atomic or list) or an expression object (but not a promise). Other objects (including classed objects) will be coerced by base::as.list.
.f	The function to be applied to each element of .x. The function is permitted, but not required, to return a promise.
...	Optional arguments to .f.

Details

promise_map processes elements of .x serially; that is, if .f(.x[[1]]) returns a promise, then .f(.x[[2]]) will not be invoked until that promise is resolved. If any such promise rejects (errors), then the promise returned by promise_map immediately rejects with that err.

Value

A promise that resolves to a list (of values, not promises).

Examples

# Waits x seconds, then returns x*10
wait_this_long <- function(x) {
  promise(\(resolve, reject) {
    later::later(\() resolve(x*10), delay = x)
  })
}

promise_map(
  list(A=1, B=2, C=3),
  wait_this_long
) |>
  then(print)

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