Hoogle Search

Within LTS Haskell 24.4 (ghc-9.10.2)

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

Exact lookup

1. lookup :: FromField a => NamedRecord -> ByteString -> Parser a

   cassava	Data.Csv

   Retrieve a field in the given record by name. The result is empty if the field is missing or if the value cannot be converted to the desired type.

2. lookup :: Ord k => k -> T k a -> Maybe a

   non-empty	Data.NonEmpty.Map

   No documentation available.

3. lookup :: IsMap map => ContainerKey map -> map -> Maybe (MapValue map)

   mono-traversable	Data.Containers

   Look up a value in a map with a specified key.

4. lookup :: (Eq k, Hashable k) => k -> HashMap k v -> Maybe v

   rio	RIO.HashMap

   Return the value to which the specified key is mapped, or Nothing if this map contains no mapping for the key.

5. lookup :: Eq a => a -> [(a, b)] -> Maybe b

   rio	RIO.List

   lookup key assocs looks up a key in an association list. For the result to be Nothing, the list must be finite.

   Examples

   >>> lookup 2 []
   Nothing
   

   >>> lookup 2 [(1, "first")]
   Nothing
   

   >>> lookup 2 [(1, "first"), (2, "second"), (3, "third")]
   Just "second"
   

6. lookup :: Ord k => k -> Map k a -> Maybe a

   rio	RIO.Map

   Lookup the value at a key in the map. The function will return the corresponding value as (Just value), or Nothing if the key isn't in the map. An example of using lookup:

   import Prelude hiding (lookup)
   import Data.Map
   
   employeeDept = fromList([("John","Sales"), ("Bob","IT")])
   deptCountry = fromList([("IT","USA"), ("Sales","France")])
   countryCurrency = fromList([("USA", "Dollar"), ("France", "Euro")])
   
   employeeCurrency :: String -> Maybe String
   employeeCurrency name = do
   dept <- lookup name employeeDept
   country <- lookup dept deptCountry
   lookup country countryCurrency
   
   main = do
   putStrLn $ "John's currency: " ++ (show (employeeCurrency "John"))
   putStrLn $ "Pete's currency: " ++ (show (employeeCurrency "Pete"))
   

   The output of this program:

   John's currency: Just "Euro"
   Pete's currency: Nothing
   

7. lookup :: Eq a => a -> [(a, b)] -> Maybe b

   rio	RIO.Prelude

   lookup key assocs looks up a key in an association list. For the result to be Nothing, the list must be finite.

   Examples

   >>> lookup 2 []
   Nothing
   

   >>> lookup 2 [(1, "first")]
   Nothing
   

   >>> lookup 2 [(1, "first"), (2, "second"), (3, "third")]
   Just "second"
   

8. lookup :: Int -> Seq a -> Maybe a

   rio	RIO.Seq

   The element at the specified position, counting from 0. If the specified position is negative or at least the length of the sequence, lookup returns Nothing.

   0 <= i < length xs ==> lookup i xs == Just (toList xs !! i)
   

   i < 0 || i >= length xs ==> lookup i xs = Nothing
   

   Unlike index, this can be used to retrieve an element without forcing it. For example, to insert the fifth element of a sequence xs into a Map m at key k, you could use

   case lookup 5 xs of
   Nothing -> m
   Just x -> insert k x m
   

9. lookup :: (HashTable h, Eq k, Hashable k) => h s k v -> k -> ST s (Maybe v)

   hashtables	Data.HashTable.Class

   Looks up a key-value mapping in a hash table. O(n) worst case, (O(1) for cuckoo hash), O(1) amortized.

10. lookup :: (HashTable h, Eq k, Hashable k) => IOHashTable h k v -> k -> IO (Maybe v)

    hashtables	Data.HashTable.IO

    See the documentation for this function in lookup.

Page 4 of many | Previous | Next