JSON output of GHC Diagnostics

[Bryan Richter]

How feasible would it be to use something like https://jsonlines.org/ ? (In
short, newline-delimited json values.) This could be in addition to any
future single-json-value output format, or as a stepping stone to that goal.

On Mon, 10 Jul 2023 at 00:46, Ben Bellick via ghc-devs <ghc-devs at haskell.org>
wrote:

> Hey everyone,
>
> I am working on a summer of Haskell project to implement JSON output of
> GHC's diagnostics. I have begun working through some of the implementation
> details but have stumbled across a few road blocks and I wanted to get some
> feedback on. Discussion surrounding the plan can be found here
> <https://github.com/haskellfoundation/tech-proposals/pull/50>.
>
> In looking through the current implementation of the -ddump-json​ flag, I
> found a note
> <https://gitlab.haskell.org/ghc/ghc/-/blob/master/compiler/GHC/Utils/Logger.hs#L392>
> which seems to indicate that a sensible (albeit invasive) approach is to
> alter the definition of LogAction
> <https://gitlab.haskell.org/ghc/ghc/-/blob/master/compiler/GHC/Utils/Logger.hs#L174>,
> reproduced below:
>
> type LogAction = LogFlags              -> MessageClass              -> SrcSpan              -> SDoc              -> IO ()
>
> So instead of taking in an SDoc​, I could alter this signature to take a
> set of Diagnostic​s, though this may introduce other complications. In
> the first place, it seems log action is processing all of the diagnostics
> one at a time, though this particular JSON output should process all of
> them at once in order to place them all in a single JSON object. So, I am
> inclined to believe that this approach may not work.
>
> Another approach could be the one originally tried and mentioned in the
> note linked above, which is to keep track of all diagnostics in an IORef​,
> and use this to produce a final JSON result at the end of GHC's execution.
> This approach seems invasive in a different way.
>
> Does anyone familiar with the handling of diagnostics throughout the
> compiler have any suggestions on how best to implement such a feature?
>
> You can find the work that I have done so far here
> <https://gitlab.haskell.org/benbellick/ghc/-/compare/master...json-diag-dump>.
> All I am really doing is checking if the new flag is set in printMessages
> <https://gitlab.haskell.org/benbellick/ghc/-/compare/master...json-diag-dump#2c680115d7e70b54a18c48be02dce2d676eaf4f8_17_18>.
> If it is, then I simply invoke the json​ function of the ToJson​
> typeclass and use that to print the diagnostic.
>
> It is currently a bit messy, but I wanted to just see if the path that I
> am taking seems fruitful or if there are any obvious hurdles that I am not
> seeing. Thanks so much!
>
> Best,
> Ben Bellick
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