Hoogle Search

Within LTS Haskell 24.38 (ghc-9.10.3)

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

Exact lookup

1. putMaybe :: MonadPut m => (a -> m ()) -> Maybe a -> m ()

   haskoin-core	Haskoin.Util.Helpers

   No documentation available.

2. untilMaybe :: Monad m => (a -> Maybe b) -> m a -> m b

   hosc	Sound.Osc.Wait

   Repeat action until f does not give Nothing when applied to result.

3. sqlAddValidCheckMaybeDowntime :: Check -> RawSQL ()

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

   Add valid check constraint. Warning: PostgreSQL acquires SHARE ROW EXCLUSIVE lock (that prevents updates) on modified table for the duration of the creation. If this is not acceptable, use sqlAddNotValidCheck and sqlValidateCheck.

4. sqlAddValidFKMaybeDowntime :: RawSQL () -> ForeignKey -> RawSQL ()

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

   Add valid foreign key. Warning: PostgreSQL acquires SHARE ROW EXCLUSIVE lock (that prevents data updates) on both modified and referenced table for the duration of the creation. If this is not acceptable, use sqlAddNotValidFK and sqlValidateFK.

5. sqlCreateIndexMaybeDowntime :: RawSQL () -> TableIndex -> RawSQL ()

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

   Create an index. Warning: if the affected table is large, this will prevent the table from being modified during the creation. If this is not acceptable, use CreateIndexConcurrentlyMigration. See https://www.postgresql.org/docs/current/sql-createindex.html for more information.

6. 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.

7. 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]
   

8. 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
   

9. 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]
   

10. 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]
    

Page 243 of many | Previous | Next