Hoogle Search

Within LTS Haskell 24.36 (ghc-9.10.3)

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

Exact lookup

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

   ghc-internal	GHC.Internal.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]
   

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

   ghc-internal	GHC.Internal.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]
   

3. minusNaturalMaybe :: Natural -> Natural -> Maybe Natural

   ghc-internal	GHC.Internal.Natural

   Natural subtraction. Returns Nothings for non-positive results.

4. naturalToWordMaybe :: Natural -> Maybe Word

   ghc-internal	GHC.Internal.Natural

   Try downcasting Natural to Word value. Returns Nothing if value doesn't fit in Word.

5. minusNaturalMaybe :: Natural -> Natural -> Maybe Natural

   ghc-internal	GHC.Internal.Numeric.Natural

   Natural subtraction. Returns Nothings for non-positive results.

6. statGetType_maybe :: Ptr CStat -> IO (Maybe IODeviceType)

   ghc-internal	GHC.Internal.System.Posix.Internals

   Unlike statGetType, statGetType_maybe will not throw an exception if the CStat refers to a unknown device type.

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

   ghc-internal	GHC.Internal.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
   

8. catMaybes :: Infinite (Maybe a) -> Infinite a

   infinite-list	Data.List.Infinite

   Keep only Just elements. This function isn't productive (e. g., head . catMaybes won't terminate), if no elements of the input list are Just.

9. mapMaybe :: (a -> Maybe b) -> Infinite a -> Infinite b

   infinite-list	Data.List.Infinite

   Apply a function to every element of an infinite list and collect Just results. This function isn't productive (e. g., head . mapMaybe f won't terminate), if no elements of the input list result in Just.

10. nextMaybeF :: Int -> (Int -> Bool) -> (Int -> Int) -> (Maybe Int -> f a) -> f a

    massiv	Data.Massiv.Core.Index

    No documentation available.

Page 176 of many | Previous | Next