Add code diagnostics for missing/extra commas, and for unmatched }, ), and ]

[wch]

For example, with this ui.R:

bootstrapPage(
  div("foo"),
  ,
  div(
    "foo"
    "bar"
  )
  123, NA, x[1],
  { 45 },
  mtcars[1,,FALSE]
))

It outputs:

> runApp('temp/commas')

Listening on http://127.0.0.1:6775
Error in parse(file, keep.source = FALSE, srcfile = src, encoding = enc) : 
  /home/winston/shiny/temp/commas/ui.R:8:5: unexpected string constant
7:     "foo"
8:     "bar"
       ^
Possible extra comma at:
5:  ,
    ^
Possible missing comma at:
8:    "bar"
      ^
Possible missing comma at:
10:  123, NA, x[1],
     ^
Possible unmatched ')' at:
13:))
    ^
Warning: Error in sourceUTF8: Error sourcing /home/winston/shiny/temp/commas/ui.R
Stack trace (innermost first):
    1: runApp [/home/winston/shiny/R/server.R#696]

If an error occurs sourcing ui.R, server.R, app.R, or global.R (anything that's sourced via sourceUTF8), shiny will run diagnoseCode on that file and print out this information.

Some issues that are worth considering:

• The first error message (with "unexpected string constant") is from R failing to parse the code; the remaining messages are from the new diagnoseCode function. It would be nice to not show R's error message and only show the messages from diagnoseCode, but I don't think that's doable in the general case because the code could have errored in any number of ways, not just the ones that the diagnoseCode function will identify.
• The diagnostics will be used for ui.R, server.R, and app.R (if an error occurs), but if any of those files source another file, that other file won't have diagnostics.
• The same applies to modules that are sourced.

[wch]

This could be pulled out into a separate function.

[jcheng5]

(For the record, @wch and I code-reviewed this in person this morning)

So lovely to get this in!

[kevinushey]

Awesome 😄