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).
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.
Value
A promise that resolves to a list (of values, not promises).
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.
Examples
# Waits x seconds, then returns x*10
wait_this_long <- function(x) {
promise(~later::later(~{
resolve(x*10)
}, delay = x))
}
promise_map(list(A=1, B=2, C=3), wait_this_long) %...>%
print()