On Thu, 2 Oct 2008, Andrew Coppin wrote:
Consider the following beautiful code:
run :: State -> Foo -> ResultSet State
run_and :: State -> Foo -> Foo -> ResultSet State run_and s0 x y = do s1 <- run s0 x s2 <- run s1 y return s2
run_or :: State -> Foo -> Foo -> ResultSet State run_or s0 x y = merge (run s0 x) (run s0 y)
That works great. Unfortunately, I made some alterations to the functionallity the program has, and now it is actually possible for 'run' to fail. When this happens, a problem should be reported to the user. (By "user" I mean "the person running my compiled application".) After an insane amount of time making my head hurt, I disocvered that the type "Either ErrorType (ResultSet State)" is actually a monad. (Or rather, a monad within a monad.) Unfortunately, this causes some pretty serious problems:
run :: State -> Foo -> Either ErrorType (ResultSet State)
You may also like to use: http://hackage.haskell.org/packages/archive/explicit-exception/0.0.1/doc/htm...