Relude.Monad.Maybe

Utility functions to work with Maybe data type as monad.

Combinators

(?:) :: Maybe a -> a -> a infixr 0

Similar to fromMaybe but with flipped arguments.

>>> readMaybe "True" ?: False
True

>>> readMaybe "Tru" ?: False
False

whenJust :: Applicative f => Maybe a -> (a -> f ()) -> f ()

Specialized version of for_ for Maybe. It's used for code readability.

Also helps to avoid space leaks: Foldable.mapM_ space leak.

>>> whenJust Nothing $ \b -> print (not b)
>>> whenJust (Just True) $ \b -> print (not b)
False

whenJustM :: Monad m => m (Maybe a) -> (a -> m ()) -> m ()

Monadic version of whenJust.

>>> whenJustM (pure Nothing) $ \b -> print (not b)
>>> whenJustM (pure $ Just True) $ \b -> print (not b)
False

whenNothing :: Applicative f => Maybe a -> f a -> f a

Performs default Applicative action if Nothing is given. Otherwise returns content of Just pured to Applicative.

>>> whenNothing Nothing [True, False]
[True,False]
>>> whenNothing (Just True) [True, False]
[True]

whenNothing_ :: Applicative f => Maybe a -> f () -> f ()

Performs default Applicative action if Nothing is given. Do nothing for Just. Convenient for discarding Just content.

>>> whenNothing_ Nothing $ putTextLn "Nothing!"
Nothing!
>>> whenNothing_ (Just True) $ putTextLn "Nothing!"

Monadic combinators

whenNothingM :: Monad m => m (Maybe a) -> m a -> m a

Monadic version of whenNothingM.

>>> whenNothingM (pure $ Just True) $ True <$ putTextLn "Is Just!"
True
>>> whenNothingM (pure Nothing) $ False <$ putTextLn "Is Nothing!"
Is Nothing!
False

whenNothingM_ :: Monad m => m (Maybe a) -> m () -> m ()

Monadic version of whenNothingM_.

>>> whenNothingM_ (pure $ Just True) $ putTextLn "Is Just!"
>>> whenNothingM_ (pure Nothing) $ putTextLn "Is Nothing!"
Is Nothing!

mapMaybeM :: Monad m => (a -> m (Maybe b)) -> [a] -> m [b]

The monadic version of the mapMaybe function.

>>> :{
evenInHalf :: Int -> IO (Maybe Int)
evenInHalf n
    | even n = pure $ Just $ n `div` 2
    | otherwise = pure Nothing
:}

>>> mapMaybeM evenInHalf [1..10]
[1,2,3,4,5]

Since: 0.6.0.0