On Tue, Sep 7, 2010 at 2:45 PM, Dimitry Golubovsky
unThrow a = unsafePerformIO $ (E.evaluate a >>= return . Right) `E.catch` (\e -> return $ Left e)
-- or perhaps the right argument of catch could be just (return . Left)?
bm2mb :: a -> Maybe a
bm2mb a = case unThrow a of Left (e::SomeException) -> Nothing Right a -> Just a
Philosophically these functions are Nasty because they violate referential transparency. In particular it's possible for the same expression to throw different exceptions each time it's run depending on how it's optimised, what other threads are doing, if the user presses ctrl-C, etc. etc. See the spoon package: http://hackage.haskell.org/package/spoon which alleviates this a little by only catching some kinds of exception, and not telling you which it caught. It still violates monotonicity (I believe), so purists will be upset, but practically it can be useful for when editing the source code to provide explicit exceptions (which is ideally what you'd do) is not an option.