I've read Jeff Newbern's tutorial on monad transformers
(http://www.nomaware.com/monads/html/index.html), but I don't grok it
yet and I can't tell how to get started with this particular
requirement, or even if I need monad transformers for this.
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
a :: Integer -> IO (Either String String)
(a :: Either String Integer) <- some_io_action_returning_integer_or_error
case a of
-- got to get from (Either String Integer) to (Either String String)
Left message -> return $ Left message
-- continue on
Right i -> do_more i
If I was using just Either I know how to get rid of the case statements:
import Control.Monad.Error
p :: Integer -> Either String Integer
q :: Integer -> Either String String
r :: Either String String
r = do
a <- p 3
b <- q a
return b
which of course is much nicer. But how do I combine the Error and IO
monads so that r could have the type IO (Either String String)?
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