Hi cafe,
I ran into a segfault while working on some database code. I eventually
traced it back to a double-finalizing of a statement (read: freeing memory
twice), which ultimately led back to switching my code to use the ContT
monad transformer. I was able to isolate this down to a minimal test case
(catch.hs); when run, it prints the line "released" twice.
In an attempt to understand what's going on, I rewrote the code to avoid the
libraries entirely (catch-simplified.hs); it didn't give me any insight into
the problem, but maybe it will help someone else.
If someone sees an obvious mistake I'm making in my usage of the bracket_
function, please let me know. Otherwise, I'd really like to get a fix for
this so I can use this library.
Thanks,
Michael
{-# LANGUAGE PackageImports #-}
import qualified "MonadCatchIO-transformers" Control.Monad.CatchIO as C
import Control.Monad.IO.Class
import Control.Monad.Trans.Cont
f :: ContT (Either String String) IO String
f = do
C.bracket_ (say "acquired") (say "released") (say "executed")
() <- error "error"
return "success"
where
say = liftIO . putStrLn
main :: IO ()
main = flip runContT (return . Right) f >>= print
{-# LANGUAGE PackageImports #-}
import qualified Control.Exception as E
cthrow :: E.SomeException -> ContT a
cthrow = cliftIO . E.throwIO
cliftIO :: IO a -> ContT a
cliftIO m = (m >>=)
ccatch :: ContT a
-> (E.SomeException -> ContT a)
-> ContT a
ccatch m f c = m c `E.catch` flip f c
type ContT a = (a -> IO Bool) -> IO Bool
creturn :: a -> ContT a
creturn a = ($ a)
cbind :: ContT a -> (a -> ContT b) -> ContT b
cbind m k c = m $ flip k c
cbind' :: ContT a -> ContT b -> ContT b
cbind' m k c = m $ const $ k c
conException :: ContT a -> ContT b -> ContT a
conException a onEx = a `ccatch` (\e -> onEx `cbind'` cthrow e)
cbracket_ ::
ContT a
-> ContT b
-> ContT c
-> ContT c
cbracket_ before after thing =
before
`cbind'`
thing `conException` after
`cbind`
\r -> after
`cbind'`
creturn r
f :: ContT String
f =
cbracket_ (say "acquired") (say "released") (say "executed")
`cbind'`
error "error"
`cbind'`
creturn "success"
where
say = cliftIO . putStrLn
main :: IO ()
main = f (return . null) >>= print
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