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Within LTS Haskell 24.50 (ghc-9.10.3)

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1. hoistMaybe :: forall (m :: Type -> Type) x a . Monad m => x -> Maybe a -> EitherT x m a

   transformers-either	Control.Monad.Trans.Either

   Hoist Maybe a into Right a.

2. liftMaybeK :: forall (m :: Type -> Type) a . Applicative m => MaybeK a -> MaybeKT m a

   unification-fd	Control.Monad.MaybeK

   Lift an MaybeK into an MaybeKT.

3. lowerMaybeK :: Applicative m => MaybeKT m a -> m (MaybeK a)

   unification-fd	Control.Monad.MaybeK

   Lower an MaybeKT into an MaybeK.

4. runMaybeK :: MaybeK a -> Maybe a

   unification-fd	Control.Monad.MaybeK

   Execute the MaybeK and return the concrete Maybe encoding.

5. runMaybeKT :: Applicative m => MaybeKT m a -> m (Maybe a)

   unification-fd	Control.Monad.MaybeK

   Execute a MaybeKT and return the concrete Maybe encoding.

6. toMaybeK :: Maybe a -> MaybeK a

   unification-fd	Control.Monad.MaybeK

   Lift a Maybe into MaybeK.

7. toMaybeKT :: forall (m :: Type -> Type) a . Applicative m => Maybe a -> MaybeKT m a

   unification-fd	Control.Monad.MaybeK

   Lift a Maybe into an MaybeKT.

8. catMaybes :: [Maybe a] -> [a]

   verset	Verset

   The catMaybes function takes a list of Maybes and returns a list of all the Just values.

   Examples

   Basic usage:

   >>> catMaybes [Just 1, Nothing, Just 3]
   [1,3]
   

   When constructing a list of Maybe values, catMaybes can be used to return all of the "success" results (if the list is the result of a map, then mapMaybe would be more appropriate):

   >>> import GHC.Internal.Text.Read ( readMaybe )
   
   >>> [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
   [Just 1,Nothing,Just 3]
   
   >>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
   [1,3]
   

9. eitherToMaybe :: Either a b -> Maybe b

   verset	Verset

   Given an Either, convert it to a Maybe, where Left becomes Nothing.

   \x -> eitherToMaybe (Left x) == Nothing
   \x -> eitherToMaybe (Right x) == Just x
   

10. fromMaybe :: a -> Maybe a -> a

    verset	Verset

    The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.

    Examples

    Basic usage:

    >>> fromMaybe "" (Just "Hello, World!")
    "Hello, World!"
    

    >>> fromMaybe "" Nothing
    ""
    

    Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

    >>> import GHC.Internal.Text.Read ( readMaybe )
    
    >>> fromMaybe 0 (readMaybe "5")
    5
    
    >>> fromMaybe 0 (readMaybe "")
    0
    

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