[Haskell-cafe] Re: How to combine Error and IO monads?

apfelmus at quantentunnel.de apfelmus at quantentunnel.de
Thu Dec 7 12:41:39 EST 2006

Cat Dancer wrote:
> I have a program that performs a series of IO operations, each which
> can result in an error or a value.  If a step returns a value I
> usually want to pass that value on to the next step, if I get an error
> I want to do some error handling but usually want to skip the
> remaining steps.

> Thus I have a lot of functions with return types like IO (Either
> String x), where x might be (), Integer, or some other useful value
> type, and a lot of case statements like 

You are on the right track. The point is that (IO (Either String a)) is
a Monad, too. This allows you to write the ever repeating case
statements once and forall:

   newtype ErrorIO a = ErrorIO (IO (Either String a))

   instance Monad ErrorIO where
       return x = return (Right x)
       f >>= g  = do
           ex <- f
           case ex of
               e@(Left _) -> return e
               Right x    -> g x

It happens that you can parametrize this on IO:

   newtype ErrorT m a = ErrorT (m (Either String a))
   type    ErrorIO a  = ErrorT IO a

   instance Monad m => Monad (ErrorT m) where ... -- same as above

And you just rediscovered monad transformers.


PS: In the special case of IO, you can also use exceptions. But using
ErrorT is better style.

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