Hoogle Search

Within LTS Haskell 24.46 (ghc-9.10.3)

Note that Stackage only displays results for the latest LTS and Nightly snapshot. Learn more.

Exact lookup

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

   unification-fd	Control.Monad.MaybeK

   Lift an MaybeK into an MaybeKT.

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

   unification-fd	Control.Monad.MaybeK

   Lower an MaybeKT into an MaybeK.

3. runMaybeK :: MaybeK a -> Maybe a

   unification-fd	Control.Monad.MaybeK

   Execute the MaybeK and return the concrete Maybe encoding.

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

   unification-fd	Control.Monad.MaybeK

   Execute a MaybeKT and return the concrete Maybe encoding.

5. toMaybeK :: Maybe a -> MaybeK a

   unification-fd	Control.Monad.MaybeK

   Lift a Maybe into MaybeK.

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

   unification-fd	Control.Monad.MaybeK

   Lift a Maybe into an MaybeKT.

7. 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]
   

8. 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
   

9. 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
   

10. mapMaybe :: (a -> Maybe b) -> [a] -> [b]

    verset	Verset

    The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list.

    Examples

    Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

    >>> import GHC.Internal.Text.Read ( readMaybe )
    
    >>> let readMaybeInt = readMaybe :: String -> Maybe Int
    
    >>> mapMaybe readMaybeInt ["1", "Foo", "3"]
    [1,3]
    
    >>> catMaybes $ map readMaybeInt ["1", "Foo", "3"]
    [1,3]
    

    If we map the Just constructor, the entire list should be returned:

    >>> mapMaybe Just [1,2,3]
    [1,2,3]
    

Page 323 of many | Previous | Next