Huh I guess I've been using maybet for so long I never realized it had been
added to the transformers library. TIL.
On Tue, Mar 4, 2014 at 2:13 AM, Yitzchak Gale
Nathan Hüsken wrote:
I want to write a function, which is basically a concatenation of functions of type "IO (Maybe a)".
David McBride wrote:
cabal install maybet
I would recommend using the standard MaybeT type, from the transformers library that is part of the Haskell Platform, unless you have a special reason to use a different library from Hackage.
David's code works fine with the transformers library, after a slight modification of the import:
import Control.Monad.Trans.Maybe
Also - using the MaybeT monad is not the only way to do this. Maybe comes with a whole collection of very convenient combinators like "maybe", "fromMaybe", etc. So you can also do the calculation entirely in just IO without very much extra noice. Choose the approach that works out nicest for your particular application.
Here is David's code, modified to do the calculations directly in the IO monad:
f1 :: IO (Maybe Int) f1 = return . Just $ 1
d2 :: Int -> IO (Maybe String) d2 = return . Just . show
blah :: IO (Maybe (Int, String)) blah = do a <- f1 b <- maybe (return Nothing) d2 a return (a,b)
If you end up using things like "maybe (return Nothing)" and "fromMaybe (return Nothing)" a lot, you can define aliases for them. Perhaps there ought to be aliases like that in the standard libraries somewhere...
Regards, Yitz _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners