sheaf pushed to branch wip/T27455 at Glasgow Haskell Compiler / GHC
Commits:
-
18b9c476
by Ben Gamari at 2026-07-31T18:53:54+02:00
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:
| 1 | +section: base
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| 2 | +issues: #27455
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| 3 | +mrs: !16274
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| 4 | +synopsis:
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| 5 | + Don't drop `ExceptionContext` in `SomeException(toException)`
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| 6 | +description:
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| 7 | + 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>`.
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| 8 | + |
| ... | ... | @@ -33,6 +33,7 @@ |
| 33 | 33 | * Evaluate backtraces for "error" exceptions at the moment they are thrown. ([CLC proposal #383](https://github.com/haskell/core-libraries-committee/issues/383))
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| 34 | 34 | * Hide implementation details when throwing exceptions in throw and throwSTM. ([CLC proposal #387](https://github.com/haskell/core-libraries-committee/issues/387))
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| 35 | 35 | * 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))
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| 36 | + * 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).
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| 36 | 37 | * Annotate `onException` continuation with `WhileHandling`. ([CLC Proposal #397](https://github.com/haskell/core-libraries-committee/issues/397))
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| 37 | 38 | * Improve error message for `Data.Char.chr`. ([CLC Proposal #384](https://github.com/haskell/core-libraries-committee/issues/384))
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| 38 | 39 |
| ... | ... | @@ -55,7 +55,7 @@ import GHC.Internal.Data.Maybe |
| 55 | 55 | import GHC.Internal.Data.Typeable (Typeable, TypeRep, cast)
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| 56 | 56 | import qualified GHC.Internal.Data.Typeable as Typeable
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| 57 | 57 | -- loop: GHC.Internal.Data.Typeable -> GHC.Internal.Err -> GHC.Internal.Exception
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| 58 | -import GHC.Internal.Base (String, Void, fmap, return, ($), (.), (++))
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| 58 | +import GHC.Internal.Base (String, Void, fmap, return, ($), (.), (++), id)
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| 59 | 59 | import GHC.Internal.Show
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| 60 | 60 | import GHC.Internal.Types (Bool(..))
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| 61 | 61 | import GHC.Internal.Exception.Context
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| ... | ... | @@ -208,7 +208,16 @@ Caught MismatchedParentheses |
| 208 | 208 | |
| 209 | 209 | -}
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| 210 | 210 | class (Typeable e, Show e) => Exception e where
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| 211 | - -- | @toException@ should produce a 'SomeException' with no attached 'ExceptionContext'.
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| 211 | + -- | 'toException' converts an exception into the existential 'SomeException'
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| 212 | + -- wrapper type.
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| 213 | + --
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| 214 | + -- In doing so, 'toException' should not /add/ an 'ExceptionContext'.
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| 215 | + --
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| 216 | + -- - In most cases, the exception does not store its own 'ExceptionContext'.
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| 217 | + -- The default implementation of 'toException' (which does not store any
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| 218 | + -- 'ExceptionContext') is suitable for these cases.
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| 219 | + -- - In the rare case that the exception itself stores an 'ExceptionContext',
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| 220 | + -- then this context should be preserved by 'toException'.
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| 212 | 221 | toException :: e -> SomeException
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| 213 | 222 | fromException :: SomeException -> Maybe e
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| 214 | 223 | |
| ... | ... | @@ -231,13 +240,11 @@ class (Typeable e, Show e) => Exception e where |
| 231 | 240 | -- | @since base-4.8.0.0
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| 232 | 241 | instance Exception Void
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| 233 | 242 | |
| 234 | --- | This drops any attached 'ExceptionContext'.
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| 243 | +-- | NB: this instance preserves the attached 'ExceptionContext'.
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| 235 | 244 | --
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| 236 | 245 | -- @since base-3.0
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| 237 | 246 | instance Exception SomeException where
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| 238 | - toException (SomeException e) =
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| 239 | - let ?exceptionContext = emptyExceptionContext
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| 240 | - in SomeException e
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| 247 | + toException = id
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| 241 | 248 | fromException = Just
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| 242 | 249 | backtraceDesired (SomeException e) = backtraceDesired e
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| 243 | 250 | displayException (SomeException e) = displayException e
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| ... | ... | @@ -4,3 +4,8 @@ foo |
| 4 | 4 | |
| 5 | 5 | HasCallStack backtrace:
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| 6 | 6 | error, called at ghc-e005.hs:12:10 in main:Main
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| 7 | + |
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| 8 | + |
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| 9 | +HasCallStack backtrace:
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| 10 | + throwIO, called at ghc\GHCi\UI.hs:1655:31 in ghc-bin-10.1.20260629-inplace:GHCi.UI
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| 11 | + |