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{-# LANGUAGE ExistentialQuantification #-}
module Bug where
import Control.Exception
import Data.Typeable
import Unsafe.Coerce
data Error
= Error Int String
| forall e . Exception e => SomeError Int e
deriving (Typeable)
fromError :: Exception e => Error -> Maybe e
fromError e@(Error _ _) = cast e
fromError (SomeError _ e) = cast e
-- {-# NOINLINE fromError #-}
instance Eq Error where
Error i s == Error i' s' = i == i' && s == s'
SomeError i e == SomeError i' e' = i == i' && show e == show e'
_ == _ = False
instance Show Error where
show _ = ""
instance Exception Error
-- newtype
data
UniquenessError = UniquenessError [((String, String), Int)]
deriving (Show, Eq)
instance Exception UniquenessError
test :: SomeException -> IO ()
test e = case fromError =<< fromException e :: Maybe UniquenessError of
Just err -> print err
_ -> pure ()
--
-- Smaller reproducer by sgraf
--
blarg :: (Int,Int) -> Int
blarg (x,y) = x+y
{-# NOINLINE blarg #-}
f :: Either Int Int -> Int
f Left{} = 0
f e = blarg (unsafeCoerce e)
blurg :: (Int -> Int) -> Int
blurg f = f 42
{-# NOINLINE blurg #-}
g :: Either Int Int -> Int
g Left{} = 0
g e = blurg (unsafeCoerce e)
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