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1. 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]
   

2. 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
   

3. 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]
   

4. 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]
   

5. readMaybe :: (Read b, StringConv a String) => a -> Maybe b

   protolude	Protolude

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

   >>> readMaybe ("123" :: Text) :: Maybe Int
   Just 123
   

   >>> readMaybe ("hello" :: Text) :: Maybe Int
   Nothing
   

6. leftToMaybe :: Either l r -> Maybe l

   protolude	Protolude.Either

   No documentation available.

7. rightToMaybe :: Either l r -> Maybe r

   protolude	Protolude.Either

   No documentation available.

8. mapMaybeNoNull :: (a -> Maybe b) -> MonoidalMap token a -> Maybe (MonoidalMap token b)

   reflex	Data.AppendMap

   Like mapMaybe but indicates whether the resulting container is empty

9. class FunctorMaybe (f :: Type -> Type)

   reflex	Reflex.Class

   Deprecated: Use Filterable from Data.Witherable instead

10. accumMaybe :: (Accumulator t f, MonadHold t m, MonadFix m) => (a -> b -> Maybe a) -> a -> Event t b -> m (f a)

    reflex	Reflex.Class

    No documentation available.

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