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Within LTS Haskell 24.2 (ghc-9.10.2)

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Exact lookup

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

   base	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"
   

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

   base	Data.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"
   

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

   base	GHC.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"
   

4. lookup :: Key -> IntMap a -> Maybe a

   containers	Data.IntMap.Internal

   Lookup the value at a key in the map. See also lookup.

5. lookup :: Key -> IntMap a -> Maybe a

   containers	Data.IntMap.Lazy

   Lookup the value at a key in the map. See also lookup.

6. lookup :: Key -> IntMap a -> Maybe a

   containers	Data.IntMap.Strict

   Lookup the value at a key in the map. See also lookup.

7. lookup :: Key -> IntMap a -> Maybe a

   containers	Data.IntMap.Strict.Internal

   Lookup the value at a key in the map. See also lookup.

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

   containers	Data.Map.Internal

   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
   

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

   containers	Data.Map.Lazy

   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
   

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

    containers	Data.Map.Strict

    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
    

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