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

7 Dec 2006

      On 12/7/06, apfelmus@quantentunnel.de  wrote:
...
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.
I think I need to explain how thoroughly clueless I am :)

I'm sure from a single example I could understand what was going on
and elaborate from there.

Let's say I want to get a line from the user, and either return an
integer or an error string using ErrorT.

  import Control.Monad.Error
  import Control.Monad.Trans

  foo :: ??
  foo = do  -- something like this?
    a <- getLine
    if length a == 1
      then return 123
      else throwError "not a single character"

  main = do
    r <- ?? foo ??
    print r -- prints Left "not a single character" or Right 123 ?