Magnus pushed to branch wip/mangoiv/9.12.5-rc4 at Glasgow Haskell Compiler / GHC Commits: d750a543 by Ben Gamari at 2026-09-04T16:43:06+02:00 base: Don't drop exception context in SomeException(toException) For reasons that are lost to time, the implementation of [CLC #200] that was merged inappropriately dropped `ExceptionContext` in the `toException` implementation given to `SomeException`. Fix this infelicity. [CLC #200]: https://github.com/haskell/core-libraries-committee/issues/200 (cherry picked from commit 2ab02c579a9625438f5281a258051cb32ba40004) - - - - - 5 changed files: - + changelog.d/T27455 - libraries/base/changelog.md - libraries/ghc-internal/src/GHC/Internal/Exception/Type.hs - testsuite/tests/ghc-e/should_fail/T18441fail7.stderr - testsuite/tests/ghc-e/should_run/ghc-e005.stderr Changes: ===================================== changelog.d/T27455 ===================================== @@ -0,0 +1,8 @@ +section: base +issues: #27455 +mrs: !16274 +synopsis: + Don't drop `ExceptionContext` in `SomeException(toException)` +description: + Previously the implementation of ``Exception(toException)`` given to `SomeException` would inappropriately drop the carried `ExceptionContext`. Now ``toException = id``, faithfully implementing the semantics proposed in :ref:`CLC Proposal #200 <https://github.com/haskell/core-libraries-committee/issues/200>`. + ===================================== libraries/base/changelog.md ===================================== @@ -2,6 +2,8 @@ ## 4.21.3.0 *June 2026* * System.Info.fullCompilerVersion: add 'since' annotation + * The implementation of `toException` in `SomeException`'s `Exception` instance no longer drops exception context, in keeping with the behavior originally proposed in [CLC Proposal #200](https://github.com/haskell/core-libraries-committee/issues/200). + ## 4.21.2.0 *March 2026* * Expose `Backtraces` constructor and fields ([CLC proposal #199](https://github.com/haskell/core-libraries-committee/issues/199), [#26049](https://gitlab.haskell.org/ghc/ghc/-/issues/26049)) ===================================== libraries/ghc-internal/src/GHC/Internal/Exception/Type.hs ===================================== @@ -206,7 +206,16 @@ Caught MismatchedParentheses -} class (Typeable e, Show e) => Exception e where - -- | @toException@ should produce a 'SomeException' with no attached 'ExceptionContext'. + -- | 'toException' converts an exception into the existential 'SomeException' + -- wrapper type. + -- + -- In doing so, 'toException' should not /add/ an 'ExceptionContext'. + -- + -- - In most cases, the exception does not store its own 'ExceptionContext'. + -- The default implementation of 'toException' (which does not store any + -- 'ExceptionContext') is suitable for these cases. + -- - In the rare case that the exception itself stores an 'ExceptionContext', + -- this context should be preserved by 'toException'. toException :: e -> SomeException fromException :: SomeException -> Maybe e @@ -229,13 +238,11 @@ class (Typeable e, Show e) => Exception e where -- | @since base-4.8.0.0 instance Exception Void --- | This drops any attached 'ExceptionContext'. +-- | NB: this instance preserves the attached 'ExceptionContext'. -- -- @since base-3.0 instance Exception SomeException where - toException (SomeException e) = - let ?exceptionContext = emptyExceptionContext - in SomeException e + toException = id fromException = Just backtraceDesired (SomeException e) = backtraceDesired e displayException (SomeException e) = displayException e ===================================== testsuite/tests/ghc-e/should_fail/T18441fail7.stderr ===================================== @@ -1,10 +1,10 @@ -<interactive>: Uncaught exception ghc-9.13-inplace:GHC.Utils.Panic.GhcException: +<interactive>: Uncaught exception ghc-9.12.4.20260710-inplace:GHC.Utils.Panic.GhcException: IO error: "Abcde" does not exist While handling IO error: "Abcde" does not exist HasCallStack backtrace: - throwIO, called at compiler/GHC/Utils/Error.hs:512:19 in ghc-9.13-inplace:GHC.Utils.Error + throwIO, called at compiler/GHC/Utils/Error.hs:512:19 in ghc-9.12.4.20260710-inplace:GHC.Utils.Error 1 ===================================== testsuite/tests/ghc-e/should_run/ghc-e005.stderr ===================================== @@ -4,3 +4,8 @@ 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 + View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d750a543f3f024b7815146e7e4d656fa... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d750a543f3f024b7815146e7e4d656fa... You're receiving this email because of your account on gitlab.haskell.org. Manage all notifications: https://gitlab.haskell.org/-/profile/notifications | Help: https://gitlab.haskell.org/help