On Sun, 5 Dec 2004, Scott Turner wrote: (snip)
Yes. Although Control.Monad.Error forces your error type to be in the Error class, that puts no constraints on what you save in the errors. If you thread your errors with the IO Monad then you would be using the monad: ErrorT YourErrorType IO When you invoke runErrorT (within the plain IO monad) it returns an Either result which delivers your error type and it can be reported however you wish. (snip)
Thanks very much! With the help of the StateT example I already had on the Haskell Wiki I managed to figure out that, to have String errors in ErrorT for code in the IO monad, I could just pass the strings to throwError, change the IO Foo functions from which errors might propagate to ErrorT String IO Foo functions, catch the error in the IO monad with runErrorT (from the Either), and where I have a function that might throw an error that uses the result of a normal IO monad function, I lift that result into the ErrorT monad. And it all seems to work. (-: -- Mark