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1. sqlDropIndexMaybeDowntime :: RawSQL () -> TableIndex -> RawSQL ()

   hpqtypes-extras	Database.PostgreSQL.PQTypes.Model.Index

   Drop an index. Warning: if you don't want to lock out concurrent operations on the index's table, use DropIndexConcurrentlyMigration. See https://www.postgresql.org/docs/current/sql-dropindex.html for more information.

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

   incipit-base	Incipit.Base

   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]
   

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

   incipit-base	Incipit.Base

   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
   

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

   incipit-base	Incipit.Base

   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]
   

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

   incipit-base	Incipit.Base

   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]
   

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

   incipit-base	Incipit.Either

   Turn Left into Just and Right into Nothing.

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

   incipit-base	Incipit.Either

   Turn Right into Just and Left into Nothing.

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

   incipit-base	Incipit.String.Reexport

   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
   

9. mapMaybeInput :: (Monoid s, Monoid s') => (s -> Maybe s') -> (s' -> Maybe s) -> Parser t s r -> Parser t s' r

   incremental-parser	Text.ParserCombinators.Incremental

   Converts a parser accepting one input type to another, just like 'mapMaybeInput except the two argument functions can demand more input by returning Nothing. If 'mapMaybeInput is defined for the two input inputs, then

   mapInput f g == mapMaybeInput (Just . f) (Just . g)
   

10. mapMaybeInput :: (Monoid s, Monoid s') => (s -> Maybe s') -> (s' -> Maybe s) -> Parser t s r -> Parser t s' r

    incremental-parser	Text.ParserCombinators.Incremental.LeftBiasedLocal

    Converts a parser accepting one input type to another, just like 'mapMaybeInput except the two argument functions can demand more input by returning Nothing. If 'mapMaybeInput is defined for the two input inputs, then

    mapInput f g == mapMaybeInput (Just . f) (Just . g)
    

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