[Haskell-cafe] IO (Either a Error) question

[Eugene Dzhurinsky]

On Sat, May 01, 2010 at 02:42:26PM -0700, Ryan Ingram wrote:
> Check out ErrorT in Control.Monad.Error
> 
> > :t ErrorT
> ErrorT :: m (Either e a) -> ErrorT e m a

At this point I am lost. I'm not sure that I do understand this type
transformation correctly. So we have some sort of monadic type m, error type e
and resut of type a. If m = IO, e - Error, a - String, than

ErrorT :: IO (Either Error String) -> ErrorT Error IO String

I can think that can be written as 

ErrorT :: IO (Either Error String) -> ErrorT Error (IO String)

Am I correct?

> So, if you can make your Error type an instance of this class, you can do this:
> runCalc = runErrorT (ErrorT (func1 p) >>= ErrorT . func2)

Sorry, I don't understand how does it work. Can you please explain the type
transformations involved here?

Thank you in advance!

-- 
Eugene N Dzhurinsky
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