relude-1.2.2.0: Safe, performant, user-friendly and lightweight Haskell Standard Library

Copyright | (c) 2016 Stephen Diehl (c) 2016-2018 Serokell (c) 2018-2023 Kowainik
License | MIT
Maintainer | Kowainik <xrom.xkov@gmail.com>
Stability | Stable
Portability | Portable
Safe Haskell | Safe
Language | Haskell2010

Relude.Monad.Maybe

Contents

- Combinators
- Monadic combinators

Description

Utility functions to work with `Maybe` data type as monad.

Synopsis

- (?:) :: Maybe a -> a -> a
- whenJust :: Applicative f => Maybe a -> (a -> f ()) -> f ()
- whenJustM :: Monad m => m (Maybe a) -> (a -> m ()) -> m ()
- whenNothing :: Applicative f => Maybe a -> f a -> f a
- whenNothing_ :: Applicative f => Maybe a -> f () -> f ()
- whenNothingM :: Monad m => m (Maybe a) -> m a -> m a
- whenNothingM_ :: Monad m => m (Maybe a) -> m () -> m ()
- mapMaybeM :: Monad m => (a -> m (Maybe b)) -> [a] -> m [b]

# 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 `whenNothing`.

    >>> 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 `whenNothing_`.

    >>> 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