How to index a function list output for use in another function?

I am using lapply() to take in a list of 2 files and compose() to apply multiple functions. The two functions being applied will load in the data from the file (list of 2 lists of dates) and then select the last element in the list.

The issue arises when I try to select the last element in each list of dates. Rather than the tail() function outputting one date, it outputs the whole list. I find that I can index the loaded data by data[[1]] for the first list and data[[2]] for the second one to get the correct final element. I don't know how to do this within the context of lapply(), compose(), and partial().

Here is the original code:

dates <- 
  list(
    from = lapply(file_last_dates,
                  purrr::compose(as.Date, purrr::partial(sum,1))),
    up_to = lapply(tamsat_retrieved_dates,
                   purrr::compose(purrr::partial(tail,n=1), st_load_rds))
    )

which outputs the following structure.

List of 2
 $ from :List of 1
  ..$ file_last_dates.rds: Date[1:1], format: "2023-01-01"
 $ up_to:List of 2
  ..$ :List of 1
  .. ..$ ST/rfe/raw/ssa/tamsat-v3.1/dekadal/retrieved_dates.rds: Date[1:1419], format: "1984-01-10" "1984-01-20" "1984-01-31" "1984-02-10" ...
  ..$ :List of 1
  .. ..$ ST/rfe/raw/ssa/tamsat-v3.1/daily/retrieved_dates.rds: Date[1:14396], format: "1984-01-01" "1984-01-02" "1984-01-03" "1984-01-04" ...

On the other hand, the following code

tail(st_load_rds(tamsat_retrieved_dates)[[1]],n=1)

and

tail(st_load_rds(tamsat_retrieved_dates)[[2]],n=1)

give me the output I want, below.

"2023-05-31"

The structure of st_load_rds(tamsat_retrieved_dates) is

List of 2 $ ST/rfe/raw/ssa/tamsat-v3.1/dekadal/retrieved_dates.rds: Date[1:1419], format: "1984-01-10" "1984-01-20" "1984-01-31" "1984-02-10" ... $ ST/rfe/raw/ssa/tamsat-v3.1/daily/retrieved_dates.rds : Date[1:14396], format: "1984-01-01" "1984-01-02" "1984-01-03" "1984-01-04" ...

and the structure of file last dates is

List of 1 $ file_last_dates.rds: Date[1:1], format: "2022-12-31"

With a structure like this ...

dates <- list(from = list(Sys.Date() + 1:3),
              to = list(Sys.Date() + 10:13)
              )

$from
$from[[1]]
[1] "2023-06-27" "2023-06-28" "2023-06-29"


$to
$to[[1]]
[1] "2023-07-06" "2023-07-07" "2023-07-08" "2023-07-09"

... you could Map or purrr::map:

## note the double brackets:
dates |> Map(f = \(li) tail(li[[1]], 1))
## $from
## [1] "2023-06-29"
## 
## $to
## [1] "2023-07-09"

... or recursively apply a function:

dates |> rapply(f = \(li) tail(li, 1)) |> as.Date(origin = '1970/01/01')
##         from           to 
## "2023-06-29" "2023-07-09" 

(Mind that, for unsorted dates, tail will return the last date while max will return the latest.)