diff --git a/MonadCatchIO-transformers.cabal b/MonadCatchIO-transformers.cabal --- a/MonadCatchIO-transformers.cabal +++ b/MonadCatchIO-transformers.cabal @@ -1,5 +1,5 @@ Name: MonadCatchIO-transformers -Version: 0.2.2.0 +Version: 0.2.2.1 Cabal-Version: >= 1.6 License: PublicDomain Description: diff --git a/src/Control/Monad/CatchIO.hs b/src/Control/Monad/CatchIO.hs --- a/src/Control/Monad/CatchIO.hs +++ b/src/Control/Monad/CatchIO.hs @@ -54,11 +54,39 @@ block = E.block unblock = E.unblock +-- | Warning: this instance is somewhat contentious. +-- +-- In the same way that the @ErrorT e@ instance may fail to perform the final +-- action, due to the "early exit" behaviour of the monad, this instance +-- may perform the final action any number of times, due to the nonlinear +-- nature of the continuation monad. +-- +-- See the mailing list message +-- +-- for an example of what can go wrong (freeing memory twice). instance MonadCatchIO m => MonadCatchIO (ContT r m) where m `catch` f = ContT $ \c -> runContT m c `catch` \e -> runContT (f e) c block = mapContT block unblock = mapContT unblock +-- | Warning: this instance is somewhat contentious. +-- +-- Note that in monads that fall under this instance (the most basic example +-- is @ErrorT e IO@), there are errors of two sorts: +-- +-- 1. exceptions, (i.e., exceptional values in the underlying IO monad); +-- +-- 2. error values of type @e@, introduced by the @ErrorT e@ part of the monad. +-- +-- The instance takes no special action to deal with errors of type 2. +-- In particular, 'bracket' will not perform its second argument, if +-- its third argument decides to "exit early" by throwing an error of type 2. +-- +-- This may or may not be what you want. +-- +-- See the mailing list thread starting with +-- +-- for some details. instance (MonadCatchIO m, Error e) => MonadCatchIO (ErrorT e m) where m `catch` f = mapErrorT (\m' -> m' `catch` \e -> runErrorT $ f e) m block = mapErrorT block