On 28/06/2010 20:02, Carl Howells wrote:
While working this weekend on the Snap web framework, I ran into a
problem. Snap implements MonadCatchIO, so I thought I could just use
bracket to handle resource acquisition/release in a safe manner.
Imagine my surprise when bracket simply failed to run the release
action sometimes.
I quickly determined the times when it doesn't run are when Snap's
monadic short-circuiting is used. I dug into the source of bracket
(in the transformers branch, though the mtl branch has the same
behavior in these cases, with slightly different code), and the reason
why quickly became obvious:
See also this recent thread on haskell-cafe:
http://www.haskell.org/pipermail/haskell-cafe/2010-June/079198.html
which concluded that the ContT instance in MonadCatchIO is broken. I
think as you say, the instances of that library are flawed when dealing
with monads that can alter the control flow.
Thanks,
Neil.
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