Re: [Haskell-beginners] Good way of combining functions of type IO (Maybe a)

I'm a beginner as well, and I explored how to use the maybeT transformer and wrote it up on my blog across a few posts. Mostly they were just for my own learning purposes, but maybe you can find something in them too? http://watchchrislearn.com/blog/2013/11/28/playing-with-the-either-monad/ http://watchchrislearn.com/blog/2013/11/30/using-the-either-monad-inside-ano... http://watchchrislearn.com/blog/2013/11/30/eithert-inside-of-io/ http://watchchrislearn.com/blog/2013/12/01/working-entirely-in-eithert/ On Mon, Mar 3, 2014 at 8:21 AM, David McBride wrote:

cabal install maybet

import Control.Monad.Maybe

f1 :: IO (Maybe Int) f1 = return . Just $ 1

d2 :: Int -> IO (Maybe String) d2 = return . Just . show

blah :: IO (Maybe (Int, String)) blah = do runMaybeT $ do a <- MaybeT f1 b <- MaybeT $ d2 a return (a,b)

Or slightly rewritten:

f1 :: MaybeT IO Int f1 = return 1 -- f1 = fail "why oh why?!?"

d2 :: Int -> MaybeT IO String d2 = return . show

blah = do runMaybeT $ do

a <- f1 b <- d2 a return (a,b)

On Mon, Mar 3, 2014 at 10:10 AM, Nathan Hüsken wrote:

...

Hey,

I want to write a function, which is basically a concatenation of functions of type "IO (Maybe a)". When they all where of type "Maybe a", no Problem I would simple use the Maybe monad.

func :: Maybe cfunc = do a <- f1 b <- d2 a ...

but now they are of type "IO (Maybe a)". Is there some way of combing these in a similar smart way?

Thanks! Nathan

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