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

You might want to make a local version of ErrorT in your library, to avoid the silly 'Error' class restriction. This is pretty easy; just copy it from the 'transformers' or 'mtl' package. On Sat, May 1, 2010 at 14:42, Ryan Ingram wrote:

Check out ErrorT in Control.Monad.Error

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:t ErrorT ErrorT :: m (Either e a) -> ErrorT e m a

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:info ErrorT instance (Monad m, Error e) => Monad (ErrorT e m)

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:info Error class Error e where    noMsg :: e    strMsg :: String -> e

So, if you can make your Error type an instance of this class, you can do this:

runCalc = runErrorT (ErrorT (func1 p) >>= ErrorT . func2)

The restriction to the typeclass Error is to allow implementation of the "fail" method in Monad.

 -- ryan

2010/5/1 Eugeny N Dzhurinsky :

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Hello!

I have some sort of strange question:

assume that there are 2 functions

func1 :: Int -> IO (Either Error String) func2 :: String -> IO (Either Error [String])

in case if there will be no IO involved, I could use Control.Monad.Either and write something like

runCalc :: Int -> IO (Either Error [String]) runCalc param = func1 param >>= func2

but with that IO stuff I can't simply do in this way. Can somebody please suggest, how to combine IO and Either monads, if that's even possible?

Thank you in advance!

-- Eugene Dzhurinsky

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