Add print.tbl_json method

[jeremystan]

Should work like print.tbl_df in dplyr and only print the two N rows (with ...s). But should also print the JSON, and use prettify(toJSON()) to do so.

[jeremystan]

This is more painful than it seems, given jsonlite interprets scalars as arrays

'{"k":"v"}' %>% fromJSON %>% toJSON == '{"k":["v"]}'
# [1] true

Also, not clear how best to format the JSON (prettify or minify? append as column or print below?)

[jeremystan]

This was a (somewhat sloppy) attempt:

print.tbl_json <- function(x, ..., n = NULL) {

  # Compute n consistent wtih dplyr::trunc_mat
  rows <- nrow(x)
  if (is.null(n)) {
    if (is.na(rows) || rows > getOption("dplyr.print_max")) {
      n <- getOption("dplyr.print_min")
    } else {
      n <- rows
    }
  }

  # Consistent with dplyr::tbl_df
  cat("tbl_json data frame ", dim_desc(x), "\n", sep = "")
  cat("\n")
  trunc_mat(x, n = n)

  # Print json formatting
  cat("\nJSON attribute:\n\n")
  take <- if (rows > n) n + 1 else n
  json <- attr(x, "JSON")[sequence(take)]
  json_char <- vapply(lapply(json, toJSON), minify, character(1))

  jdf <- data.frame(
    JSON = json_char,
    stringsAsFactors = FALSE)
  trunc_mat(jdf, n = n)

}