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sheaf pushed to branch wip/T27455 at Glasgow Haskell Compiler / GHC

Commits:

b8958d1e

by Ben Gamari at 2026-07-03T13:50:19+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

 +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
@@ -231,13 +231,11 @@ 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
++