Proposal: add ifM and whenM to Control.Monad

[Mario Pastorelli]

+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 at 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
>
> andreas.abel at gu.se
> http://www2.tcs.ifi.lmu.de/~abel/
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