R version 4.5.0 RC (2025-04-03 r88103 ucrt) -- "How About a Twenty-Six"
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Platform: x86_64-w64-mingw32/x64

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> library(RJSONIO)
> jtxt = '[ 1, "abc", "xyz"]'
> jdate = '[ 1, "/Date(1335746208)/", "xyz"]'
> jnewdate = '[ 1, "/new Date(1335746208)/", "xyz"]'
> 
> 
> 
> a = fromJSON(jtxt, stringFun = I(getNativeSymbolInfo("dummyStringOperation")$address))
> b = fromJSON(jtxt, stringFun = I(getNativeSymbolInfo("dummyStringOperation")))
> c = fromJSON(jtxt, stringFun = I("dummyStringOperation"))
> d = fromJSON(jtxt, stringFun = structure("dummyStringOperation", class = "NativeStringRoutine"))
> 
> e = fromJSON(jtxt, stringFun = I("dummyStringOperation"), simplify = TRUE)
> 
> 
> ans = fromJSON(jtxt, stringFun = structure("R_json_dateStringOp", class = "SEXPRoutine"))
> sapply(ans, class) == c("numeric", "character", "character")
[1] TRUE TRUE TRUE
> ans = fromJSON(jtxt, stringFun = "R_json_dateStringOp")
> 
> ans = fromJSON(jtxt, stringFun = structure("R_json_dateStringOp", class = "SEXPRoutine"), simplify = TRUE)
> 
>  # process jdate
> ans = fromJSON(jdate, stringFun = structure("R_json_dateStringOp", class = "SEXPRoutine"))
> ans = fromJSON(jdate, stringFun = "R_json_dateStringOp")
> 
> 
>   # process strings by just returning them.
> fromJSON(jtxt, stringFun = function(val) val)
[[1]]
[1] 1

[[2]]
[1] "abc"

[[3]]
[1] "xyz"

> 
>   #  process strings by prefixing them with  xxx_
> fromJSON(jtxt, stringFun = function(val) sprintf("xxx_%s", val))
[[1]]
[1] 1

[[2]]
[1] "xxx_abc"

[[3]]
[1] "xxx_xyz"

> 
> jtxt = '[ "1", "2.3", "3.1415"]'  # all numeric but in "" 
> ans = fromJSON(jtxt)
> stopifnot(is.character(ans))
> 
>  # convert all of the strings to numeric!
> ans = fromJSON(jtxt, stringFun = function(val) as.numeric(val))
> stopifnot(is.numeric(ans))
> 
>  # Now convert them all to TRUE as logicals
> ans = fromJSON(jtxt, stringFun = function(val) TRUE)
> stopifnot(is.logical(ans))
> 
> 
> #
> jtxt = '[ 1, "/new Date(12312313)", "/Date(12312313)"]'
> ans = fromJSON(jtxt)
> 
> ans = fromJSON(jtxt, stringFun = "R_json_dateStringOp", simplify = FALSE)
> stopifnot(all(mapply(is, ans, c("numeric", "POSIXct", "POSIXct"))))
> 
> 
> 
> proc.time()
   user  system elapsed 
   0.15    0.04    0.18