On 04/19/10 02:15, Anders Kaseorg wrote:
I would be very happy to get the simpler interface to work, because it’s
Haskell 98. However, if I write
joinIO m = morphIO (\w -> m>>= w)
morphIO' f = joinIO (f return)
and define catch using morphIO' instead of morphIO:
m `catch` h = morphIO $ \w -> w m `Control.Exception.catch` \e -> w (h e)
m `catch'` h = morphIO' $ \w -> w m `Control.Exception.catch` \e -> w (h
e)
then catch' fails to actually catch anything:
*Main> throwIO NonTermination `catch` \NonTermination -> return "moo"
"moo"
*Main> throwIO NonTermination `catch'` \NonTermination -> return "moo"
*** Exception:<<loop>>
Am I doing something wrong?
Well, let's see what happens if we apply it to the fairly easy
> instance MonadMorphIO IO where
> morphIO f = f id
then
joinIO m = morphIO (\w -> m >>= w)
= (\w -> m >>= w) (id) = m >>= id = join m
morphIO' f = joinIO (f return) = join (f return)
morphIO = f id
m `catch` h = morphIO $ \w -> w m `Control.Exception.catch` \e -> w (h e)
[w = id]
= id m `Control.Exception.catch` \e -> id (h e)
m `catch'` h = morphIO' $ \w -> w m `Control.Exception.catch` \e -> w
(h e)
[w = return]
= join (return m `Control.Exception.catch` \e -> return (h e))
Do you see the difference? The effects are sequenced in different
places. The return/join pair moves all the effects *outside* the
operations such as catch... thus defeating the entire purpose of morphIO.
-Isaac
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