Hello tsuraan,
Most often, when we multi-pattern-match on the return value of a monadic
computation, we talk about Maybe or Either or [], and I often find
myself doing this:
someIO1 :: IO (Maybe A)
someIO2 :: IO (Either A B)
result1 <- someIO1 >>= maybe ...
result2 <- someIO2 >>= either ...
There are many ways of encoding this more nicely. My personal way is to
use the proper monad transformers for the purpose. For many of these
situations I have written convenient combinators in the 'contstuff'
package. I found especially the 'liftF' function very useful in these
cases:
liftF :: (LiftFunctor t, Monad m) => m (InnerFunctor t a) -> t m a
Example instances:
liftF :: Monad m => m [a] -> ChoiceT r i m a
liftF :: Monad m => m (Either e a) -> EitherT r e m a
liftF :: Monad m => m (Maybe a) -> MaybeT r m a
That way instead of checking each return value individually you would
just write:
result <- evalMaybeT $ do
x <- liftF someMaybeIO
y <- liftF (someOtherMaybeIO x x)
return (Result x y)
Greets,
Ertugrul
tsuraan
In my code, I'm doing this quite a lot:
x <- someIO case x of Opt1 -> ...
Having a line for extracting the value from the IO (or STM) and then acting on the value seems unnatural. Is there a more concise way to do this? This code:
case someIO of Opt1 -> ...
Doesn't work, but is there something like that, that is valid?
-- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/