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

John Millikin jmillikin at gmail.com
Sat May 1 17:49:27 EDT 2010


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 <ryani.spam at gmail.com> wrote:
> Check out ErrorT in Control.Monad.Error
>
>> :t ErrorT
> ErrorT :: m (Either e a) -> ErrorT e m a
>
>> :info ErrorT
> instance (Monad m, Error e) => Monad (ErrorT e m)
>
>> :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 <bofh at redwerk.com>:
>> 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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>>
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