Error message infrastructure

I have just spent a couple of hours wrestling with the new error message infrastructure for the typechecker. Here are some thoughts and questions.

To be clear, the direction of travel is great -- this is all about detail.

1. Surely we should have

   instance Outputable e => Outputable (Messages e)
   instance Outputable e => Outputable (MsgEnvelope e)

   I needed it today, and although took me over an hour of faff to figure out how to do it, it's not so hard

   instance Diagnostic e => Outputable (Messages e) where
       ppr msgs = braces (vcat (map ppr_one (bagToList (getMessages msgs))))
          where
            ppr_one :: MsgEnvelope e -> SDoc
            ppr_one envelope = ppr_diag (errMsgDiagnostic envelope)
   
            ppr_diag :: e -> SDoc
            ppr_diag diag = vcat [ ppr (diagnosticReason diag)
                                 , nest 2 (vcat (unDecorated (diagnosticMessage diag))) ]

   This could be improved.

2. This is newtype poorly named:

   newtype Messages e = Messages { getMessages :: Bag (MsgEnvelope e) }

   Either the newtype should be MsgEnvelopes e or the contents should be Bag (Message e).

3. We have in GHC.Tc.Errors.Types:

   data TcRnMessage where
      TcRnUnknownMessage :: (Diagnostic a, Typeable a) => a -> TcRnMessage
      TcRnMessageWithInfo :: !UnitState -> !TcRnMessageDetailed -> TcRnMessage
      TcRnSolverReport :: [ReportWithCtxt] -> DiagnosticReason -> [GhcHint] -> TcRnMessage
      ...lots more error-secific-constructors...
   
   data TcRnMessageDetailed
      = TcRnMessageDetailed !ErrInfo !TcRnMessage

   Questions

   • Can't we just inline TcRnMessageDetailed into TcRnMessageWithInfo?
   • How would I choose between TcRnUnknownMessage and TcRnMessageWithInfo and an error-specific constructor?

4. Also in GHC.Tc.Errors.Types:

   data ReportWithCtxt = ReportWithCtxt ReportErrCtxt TcReportMsg
   
   data TcReportMsg where
      ...lots *more* error-specific constructors....

   • Why do we need TcRnMessage and TcReportMsg? Ah.. maybe those are solver-specific errors? But then calll it TcSolverMessage etc.
   • And put all the solver-specific stuff in TcRnSolverReport; at the moment TcRnInaccessibleCode sticks out like a sore thumb.
   • Why does TcRnSolverReport take a list of solver errors? It really has one error, plus a list of supplementary information, a bit like your various "WithInfo" variants, doesn't it?

Generally, the data types are so large that I found it really helpful to write down the above sketches, just to keep them in my head. We might consider doing that in some way.