[Haskell-cafe] Question on a common pattern
Ertugrul Soeylemez
es at ertes.de
Tue Mar 15 16:21:37 CET 2011
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 <tsuraan at gmail.com> wrote:
> 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/
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