sheaf pushed to branch wip/T27455 at Glasgow Haskell Compiler / GHC Commits: d955fe1e by Ben Gamari at 2026-07-31T18:56:19+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 - - - - - 4 changed files: - + changelog.d/T27455 - libraries/base/changelog.md - libraries/ghc-internal/src/GHC/Internal/Exception/Type.hs - 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 ===================================== @@ -33,6 +33,7 @@ * Evaluate backtraces for "error" exceptions at the moment they are thrown. ([CLC proposal #383](https://github.com/haskell/core-libraries-committee/issues/383)) * Hide implementation details when throwing exceptions in throw and throwSTM. ([CLC proposal #387](https://github.com/haskell/core-libraries-committee/issues/387)) * Change `hIsReadable` and `hIsWritable` such that they always throw a respective exception when encountering a closed or semi-closed handle, not just in the case of a file handle. ([CLC proposal #371](github.com/haskell/core-libraries-committee/issues/371)) + * 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). * Annotate `onException` continuation with `WhileHandling`. ([CLC Proposal #397](https://github.com/haskell/core-libraries-committee/issues/397)) * Improve error message for `Data.Char.chr`. ([CLC Proposal #384](https://github.com/haskell/core-libraries-committee/issues/384)) ===================================== libraries/ghc-internal/src/GHC/Internal/Exception/Type.hs ===================================== @@ -55,7 +55,7 @@ import GHC.Internal.Data.Maybe import GHC.Internal.Data.Typeable (Typeable, TypeRep, cast) import qualified GHC.Internal.Data.Typeable as Typeable -- loop: GHC.Internal.Data.Typeable -> GHC.Internal.Err -> GHC.Internal.Exception -import GHC.Internal.Base (String, Void, fmap, return, ($), (.), (++)) +import GHC.Internal.Base (String, Void, fmap, return, ($), (.), (++), id) import GHC.Internal.Show import GHC.Internal.Types (Bool(..)) import GHC.Internal.Exception.Context @@ -208,7 +208,16 @@ 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 @@ -231,13 +240,11 @@ 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_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/d955fe1e7197fecca6fd319ff55318e8... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d955fe1e7197fecca6fd319ff55318e8... 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