+1 for unlessM On 04/20/2014 09:38 PM, Andreas Abel wrote:
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+1 for both, plus
unlessM :: (Monad m) => m Bool -> m () -> m () unlessM mc ma = ifM mc (return ()) ma
(See, for instance, https://hackage.haskell.org/package/Agda-2.3.2.2/docs/Agda-Utils-Monad.html .)
On 20.04.2014 21:10, Mario Pastorelli wrote:
I would like to propose the addition of two new combinators to Control.Monad:
ifM :: (Monad m) => m Bool -> m a -> m a -> m a whenM :: (Monad m) => m Bool -> m () -> m ()
The reason is that when you work in a `Monad m` the first argument of `if` and `when` is often a `m Bool` and not a `Bool`. In those cases, you have to write:
monadicOperationReturningBool >>= \b -> when b doAnotherMonadicOperation
or
monadicOperationReturningBool >>= flip when doAnotherMonadicOperation
to accomplish what you want to do. If you use the do-notation this is less terrible but you still need to assign a name to the boolean value. Take for example:
f = do dirDoesntExist <- not <$> doesDirectoryExist path when dirDoesntExist $ do putStrLn $ "Creating directory " ++ path createDirectory path
in this snippet, dirDoesntExist is completely useless and its only purpose it to be used in the next expression. With the new combinators you could write:
f = whenM (not <$> doesDirectoryExists path) $ do putStrLn $ "Creating directory " ++ path createDirectory path
Many libraries on hackage already include these functions (I often use Control.Conditional). For a list see http://holumbus.fh-wedel.de/hayoo/hayoo.html?query=whenM&start=0. _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries
- -- Andreas Abel <>< Du bist der geliebte Mensch.
Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden
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