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Within LTS Haskell 24.32 (ghc-9.10.3)

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  1. maybeManageDebug :: ManageHook

    xmonad-contrib XMonad.Hooks.ManageDebug

    manageDebug only if the user requested it with debugNextManagedWindow.

  2. maybeToDefinite :: (Monoid a, Functor m) => m (Maybe a) -> m a

    xmonad-contrib XMonad.Hooks.ManageHelpers

    converts MaybeManageHooks to ManageHooks

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

    base Data.Maybe

    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]

  4. fromMaybe :: a -> Maybe a -> a

    base Data.Maybe

    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

  5. listToMaybe :: [a] -> Maybe a

    base Data.Maybe

    The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.

    Examples

    Basic usage: 
    >>> listToMaybe []
Nothing

    >>> listToMaybe [9]
Just 9

    >>> listToMaybe [1,2,3]
Just 1
    Composing maybeToList with listToMaybe should be the identity on singleton/empty lists: 
    >>> maybeToList $ listToMaybe [5]
[5]

>>> maybeToList $ listToMaybe []
[]
    But not on lists with more than one element: 
    >>> maybeToList $ listToMaybe [1,2,3]
[1]

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

    base Data.Maybe

    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]

  7. bitSizeMaybe :: Bits a => a -> Maybe Int

    base Data.Bits

    Return the number of bits in the type of the argument. The actual value of the argument is ignored. Returns Nothing for types that do not have a fixed bitsize, like Integer.

  8. minusNaturalMaybe :: Natural -> Natural -> Maybe Natural

    base Numeric.Natural

    Natural subtraction. Returns Nothings for non-positive results.

  9. readMaybe :: Read a => String -> Maybe a

    base Text.Read

    Parse a string using the Read instance. Succeeds if there is exactly one valid result. 

    >>> readMaybe "123" :: Maybe Int
Just 123

    >>> readMaybe "hello" :: Maybe Int
Nothing

  10. bitSizeMaybe :: Bits a => a -> Maybe Int

    base GHC.Bits

    Return the number of bits in the type of the argument. The actual value of the argument is ignored. Returns Nothing for types that do not have a fixed bitsize, like Integer.

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