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Simon Jakobi pushed to branch wip/sjakobi/T27847-ghci-rethrow-backtrace at Glasgow Haskell Compiler / GHC

Commits:

b5957144

by Simon Jakobi at 2026-09-25T01:11:21+02:00

ghc -e: Print uncaught exceptions without their context

Since 2ab02c579a9, exceptions rethrown inside GHCi keep collecting
backtraces and WhileHandling annotations, so a failing `ghc -e` or
runghc script printed extra "HasCallStack backtrace" blocks pointing
into GHC's own sources (#27847).

By the time an exception reaches the top-level handler, its context
only describes GHC-internal frames: the user program's own context is
already part of the message text. So drop the context before printing.
This also removes the WhileHandling noise from prettyPrintGhcErrors
that predates 2ab02c579a9.

Fixes #27847.

Assisted-by: Claude Opus 5.5

5 changed files:

compiler/GHC/Utils/Exception.hs
ghc/GHCi/UI.hs
libraries/ghci/GHCi/Message.hs
testsuite/tests/ghc-e/should_fail/T18441fail7.stderr
testsuite/tests/ghc-e/should_run/ghc-e005.stderr

Changes:

compiler/GHC/Utils/Exception.hs

@@ -11,6 +11,7 @@ import GHC.Prelude.Basic
  import GHC.IO (catchException)
  import Control.Exception as CE hiding (assert)
 +import Control.Exception.Context (emptyExceptionContext)
  import Control.Monad.IO.Class
  import Control.Monad.Catch
@@ -24,4 +25,8 @@ handleIO = flip catchIO
  tryIO :: IO a -> IO (Either IOException a)
  tryIO = CE.try
 +-- | Remove the context, such as backtraces and annotations, from an exception.
 +dropExceptionContext :: SomeException -> SomeException
 +dropExceptionContext e = toException (ExceptionWithContext emptyExceptionContext e)
++
  type ExceptionMonad m = (MonadCatch m, MonadThrow m, MonadMask m, MonadIO m)

ghc/GHCi/UI.hs

@@ -1079,8 +1079,12 @@ runGHCi paths maybe_exprs = do
                              -- current progname in the exception text:
                              -- <progname>: <exception>
                              liftIO $ withProgName (progname st)
 -                                   $ topHandler e
                                     -- this used to be topHandlerFastExit, see #2228
 +                                   $ topHandler
 +                                   -- The program's own context is already part of the
 +                                   -- message (see toSerializableException). The rest
 +                                   -- comes from GHC itself (#27847).
 +                                   $ dropExceptionContext e
              runInputTWithPrefs defaultPrefs defaultSettings $ do
                  -- make `ghc -e` exit nonzero on failure, see #7962, #9916, #17560, #18441
                  _ <- runCommands' hdle

libraries/ghci/GHCi/Message.hs

@@ -466,8 +466,7 @@ data LoadedDLL
  -- * UserInterrupt has a special meaning
  -- * In ghc -e, exitWith should exit with the appropriate exit code
  --
 --- So all we need to do is distinguish UserInterrupt and ExitCode, and
 --- all other exceptions can be represented by their 'show' string.
 +-- So all we need to do is distinguish UserInterrupt and ExitCode.
  --
  data SerializableException
    = EUserInterrupt

testsuite/tests/ghc-e/should_fail/T18441fail7.stderr

  <interactive>: Uncaught exception ghc-10.1-inplace:GHC.Utils.Panic.GhcException:
  IO error:  "Abcde" does not exist
+-
 -While handling ghc-10.1-inplace:GHC.Utils.Panic.GhcException:
 -  |
 -  | IO error:  "Abcde" does not exist
 -  |
 -  | HasCallStack backtrace:
 -  |   throw, called at compiler/GHC/Utils/Panic.hs:180:21 in ghc-10.1-inplace:GHC.Utils.Panic
 -  |   throwGhcException, called at ghc/GHCi/UI.hs:2851:21 in ghc-bin-10.1.20260801-inplace:GHCi.UI
+-
 -HasCallStack backtrace:
 -  throwIO, called at compiler/GHC/Utils/Error.hs:513:19 in ghc-10.1-inplace:GHC.Utils.Error
+-

testsuite/tests/ghc-e/should_run/ghc-e005.stderr

@@ -4,8 +4,3 @@ foo
  HasCallStack backtrace:
    error, called at ghc-e005.hs:12:10 in main:Main
+-
+-
 -HasCallStack backtrace:
 -  throwIO, called at ghc\GHCi\UI.hs:1655:31 in ghc-bin-10.1.20260629-inplace:GHCi.UI
+-