Draft: Print exception type metadata in default handler

Prerequisites

• Merge test fix MRs
  • ghc/packages/array!27 (merged)
  • https://github.com/haskell/stm/pull/81
• Update array submodule here
• Update stm submodule here
  • Update gitlab stm mirror

Intro

See https://github.com/haskell/core-libraries-committee/issues/231.

The linked issue requests metadata in the printed output for exceptions. For example:

main :: IO ()
main = throwBlocked

throwBlocked :: IO ()
throwBlocked = throwIO BlockedIndefinitelyOnMVar

Main: thread blocked indefinitely in an MVar operation

Package: base
Module: GHC.IO.Exception
Name: BlockedIndefinitelyOnMVar

With the future changes to use displayException with callstacks (https://github.com/haskell/core-libraries-committee/issues/198), this may look like:

Main: thread blocked indefinitely in an MVar operation
HasCallStack backtrace:
CallStack (from HasCallStack):
  collectBacktraces, called at libraries/base/GHC/Exception.hs:67:13 in base:GHC.Exception
  toExceptionWithBacktrace, called at libraries/base/GHC/IO.hs:278:11 in base:GHC.IO
  throwIO, called at Main.hs:15:16 in main:Main

Package: base
Module: GHC.IO.Exception
Name: BlockedIndefinitelyOnMVar

Notes

Note that this is rebased on top of the new exception context branch: wip/exception-context. This is done for diff / future-rebasing purposes. I am not literally asking for this to be merged into the target branch, and I very much do not want to create any distractions for anyone working on that issue 🙂.

2024-25-01: This is now branched off master, so it can be merged promptly.

Questions

1. I am not overly attached to the current output format: feedback welcome. For instance, we could have a compact form like:

   λ. ./Main 
   Main: thread blocked indefinitely in an MVar operation
   HasCallStack backtrace:
   CallStack (from HasCallStack):
     collectBacktraces, called at libraries/base/GHC/Exception.hs:67:13 in base:GHC.Exception
     toExceptionWithBacktrace, called at libraries/base/GHC/IO.hs:278:11 in base:GHC.IO
     throwIO, called at Main.hs:33:16 in main:Main
   
   Type: base:GHC.IO.Exception:BlockedIndefinitelyOnMVar

   Also, the string concatenation is pretty gross, though maybe the simplicity is preferred, as we are already working with strings, and performance is unlikely to matter here.

Thanks!