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Within LTS Haskell 24.36 (ghc-9.10.3)

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  1. scatMaybes :: Stream r ix (Maybe a) => Array r ix (Maybe a) -> Vector DS a

    massiv Data.Massiv.Vector

    Keep all Maybes and discard the Nothings.

    Examples

  2. simapMaybe :: Stream r ix a => (ix -> a -> Maybe b) -> Array r ix a -> Vector DS b

    massiv Data.Massiv.Vector

    Similar to smapMaybe, but map with an index aware function.

    Examples

  3. simapMaybeM :: forall r ix a b f . (Stream r ix a, Applicative f) => (ix -> a -> f (Maybe b)) -> Array r ix a -> f (Vector DS b)

    massiv Data.Massiv.Vector

    Similar to smapMaybeM, but map with an index aware function.

    Examples

  4. smapMaybe :: Stream r ix a => (a -> Maybe b) -> Array r ix a -> Vector DS b

    massiv Data.Massiv.Vector

    Apply a function to each element of the array, while discarding Nothing and keeping the Maybe result.

    Examples

  5. smapMaybeM :: forall r ix a b f . (Stream r ix a, Applicative f) => (a -> f (Maybe b)) -> Array r ix a -> f (Vector DS b)

    massiv Data.Massiv.Vector

    Similar to smapMaybe, but with the Applicative function. Similar to mapMaybe id $ mapM f arr

    Examples

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

    protolude Protolude

    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.

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

    protolude Protolude

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

    protolude Protolude

    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

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

    protolude Protolude

    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]

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

    protolude Protolude

    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]

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