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.