On Fri, Oct 15, 2010 at 10:22 AM,
Michael Snoyman wrote:
I would prefer if the test read as:
test33 = fmap (== Left "throwError") $ test3c (throwError "throwError" :: ErrorT String IO String)
Which never in fact returns True. Or, more to the point, the test is never even called, since the runtime exception prevents it.
If you prefer the raised exception to be reflected back into the monad it came from, that can be arranged. I have updated
http://okmij.org/ftp/Haskell/CaughtMonadIO.lhs
to use your test:
test331 = fmap (== Left (show "throwError")) $ test3c (throwError "throwError" :: ErrorT String IO String)
*CaughtMonadIO> test331 sequel called True
The `show' is the artifact of `reconciling' Error and Exception classes. The class Error doesn't seem very informative; one may wonder if it is needed given that we already have Exception. If in your real code, the argument of throwError is actually an Exception, the show hack can be eliminated, where it is mentioned in the instance CaughtMonadIO (ErrorT e m). I can do the adjustment if you post more details about the desired functionality.
By the way, I completely agree that the Error typeclass is not very useful, and I wish it would just disappear. I wrote the neither package[1]- the modified error monad I keep mentioning- in large part to avoid the Error typeclass. The other reason was the oprhan Monad Either instance. To the point at hand: I'm aware that you can promote the Error monad's error type into a runtime exception, catch it, and pull it back down. My point is that you shouldn't have to: there are perfectly valid definitions of finally for the error monad that don't require any of this trickery. Additionally, sometimes the error type cannot (reasonably) be promoted to an exception, eg ErrorT (a -> b) m. We'd have to start mucking around with dummy Show instances/blind wrappers. I just finished writing up a post describing the MonadInvertIO approach[2], which I think is a more appropriate solution to the problem. It doesn't involve any runtime exception trickery, and works for a number of other use cases, including memory allocation. I would appreciate any critiques of my ideas. Michael [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/neither [2] http://docs.yesodweb.com/blog/invertible-monads-exceptions-allocations/#mona... (links straight to the appropriate section, skips the long buildup)