R version 4.5.0 alpha (2025-03-25 r88054 ucrt)
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Platform: x86_64-w64-mingw32/x64

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> library(RJSONIO)
> setClass("A", representation(x = "integer", abc = "character"),
+           prototype = list(x = 1:10, abc = c("Jan", "Feb", "Mar")))
> a = new("A")
> 
> toJSON(a)
[1] "{ \"x\": [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ],\"abc\": [ \"Jan\", \"Feb\", \"Mar\" ] }"
> 
> toJSON(list(a, a))
[1] "[\n { \"x\": [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ],\"abc\": [ \"Jan\", \"Feb\", \"Mar\" ] },\n{ \"x\": [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ],\"abc\": [ \"Jan\", \"Feb\", \"Mar\" ] } \n]"
> 
> 
> proc.time()
   user  system elapsed 
   0.26    0.09    0.32