Similar to purrr::reduce (left fold), but the function .f is permitted to return a promise. promise_reduce will wait for any returned promise to resolve before invoking .f with the next element; in other words, execution is serial. .f can return a promise as output but should never encounter a promise as input (unless .x itself is a list of promises to begin with, in which case the second parameter would be a promise).

promise_reduce(.x, .f, ..., .init)



A vector or list to reduce. (Not a promise.)


A function that takes two parameters. The first parameter will be the "result" (initially .init, and then set to the result of the most recent call to func), and the second parameter will be an element of .x.


Other arguments to pass to .f


The initial result value of the fold, passed into .f when it is first executed.


A promise that will resolve to the result of calling .f on the last element (or .init if .x had no elements). If any invocation of .f

results in an error or a rejected promise, then the overall promise_reduce promise will immediately reject with that error.


# Returns a promise for the sum of e1 + e2, with a 0.5 sec delay
slowly_add <- function(e1, e2) {
  promise(~later::later(~resolve(e1 + e2), delay = 0.5))

# Prints 55 after a little over 5 seconds
promise_reduce(1:10, slowly_add, .init = 0) %...>% print()