Apply call-back function and return values when hander is not NULL

[koheiw]

When I was looking for a way to get one big list from aggreagate(), I saw #236. I think handler should work in this way.

library(mongolite)
con <- mongo("mtcars", url =
               "mongodb+srv://readwrite:test@cluster0-84vdt.mongodb.net/test")
if(con$count() > 0) con$drop()
con$insert(mtcars)
#> List of 5
#>  $ nInserted  : num 32
#>  $ nMatched   : num 0
#>  $ nRemoved   : num 0
#>  $ nUpserted  : num 0
#>  $ writeErrors: list()

# data.frame
con$aggregate('[{"$match": {}}, {"$project": {"mpg": 1, "cyl": 1}}]')[1:2,] 
#>                        _id mpg cyl
#> 1 656a87c6ab879540600b9221  21   6
#> 2 656a87c6ab879540600b9222  21   6

# list
con$aggregate('[{"$match": {}}, {"$project": {"mpg": 1, "cyl": 1}}]', handler = as.list)[1:2] 
#> [[1]]
#> [[1]]$`_id`
#> [1] "656a87c6ab879540600b9221"
#> 
#> [[1]]$mpg
#> [1] 21
#> 
#> [[1]]$cyl
#> [1] 6
#> 
#> 
#> [[2]]
#> [[2]]$`_id`
#> [1] "656a87c6ab879540600b9222"
#> 
#> [[2]]$mpg
#> [1] 21
#> 
#> [[2]]$cyl
#> [1] 6

^{Created on 2023-12-02 with reprex v2.0.2}