fromJSON: Manually set how null is handled (like jsonlite)

[ChingChuan-Chen]

Can you include an option to setup what null is parsed in JSON file?
I would prefer to convert null to NA rather than NULL.

Ex:

fromJSON("[123,null]", null = NA) 
# [1] 123 NA

rather than

[[1]]
123
[[2]]
NULL

ref: jeroen/jsonlite#70

[ChingChuan-Chen]

It can be solved by:

do.call(c, jsonlite:::null_to_na(rjson::fromJSON("[123,null]")))

Although it is stupid to use two JSON-related packages, it is two times faster than jsonlite::fromJSON.

zz <- replicate(1e4L, "[12,11,10,9,null,7,6]")
library(microbenchmark)
microbenchmark(
  jsonlite = Vectorize(jsonlite::fromJSON, USE.NAMES = FALSE)(zz),
  rjson = Vectorize(function(x){
    do.call(c, jsonlite:::null_to_na(rjson::fromJSON(x)))
  }, USE.NAMES = FALSE)(zz), times = 20L
)
## Unit: milliseconds
##      expr      min       lq     mean   median       uq      max neval
##  jsonlite 803.8155 807.4785 820.5294 811.1097 820.2690 918.9634    20
##     rjson 244.1544 247.5007 250.6781 248.8216 251.7142 286.0120    20