On Mon, Jun 21, 2010 at 7:04 AM, Neil Brown <nc...@kent.ac.uk> wrote: > > Here's my speculation, based on glancing at the libraries involved: I > believe the reason for this may be the MonadCatchIO instance for ContT: > > === > instance MonadCatchIO m => MonadCatchIO (ContT r m) where > m `catch` f = ContT $ \c -> runContT m c `catch` \e -> runContT (f e) c > === > > To my eye, that code takes the continuation to run after the block, c (which > in your case involves the after-action from bracket_, and then the error), > and runs that inside the catch block. This causes a successful completion > of bracket_ (first release), followed by the error, which triggers the catch > block which then runs the final actions (second release) and rethrows the > error. Does that sound possible to anyone else?
Sounds possible to me. ContT does not play well with control operations from other monads. Most people would expect, e.g., lift m `catch` lift . f = lift (m `catch` f), but ContT does not have such an operation. If you really want explicit continuations and exceptions, you need a monad written specifically for that. -- Dave Menendez <d...@zednenem.com> <http://www.eyrie.org/~zednenem/> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
