map Include in Haskell

In Haskell, we can check if a hash includes a key using the Data.HashMap.Lazy.lookup function which returns a Maybe value. A Just value indicates that the key exists in the hash and contains the corresponding value, while a Nothing value indicates that the key does not exist in the hash.

Here is an example of how to check if a key exists in a hash in Haskell:

import qualified Data.HashMap.Lazy as Map
-- Define a hash

hash = Map.fromList [("apple", 1), ("banana", 2), ("orange", 3)]
-- Check if "apple" exists in the hash

appleExists = case Map.lookup "apple" hash of

               Just value -> True

               Nothing -> False

               

-- Check if "watermelon" exists in the hash

watermelonExists = case Map.lookup "watermelon" hash of

                   Just value -> True

                   Nothing -> False

                   

-- Print the results

main = do

    putStrLn $ "Does 'apple' exist in the hash? " ++ show appleExists

    putStrLn $ "Does 'watermelon' exist in the hash? " ++ show watermelonExists

The above code will output:

Does 'apple' exist in the hash? True
Does 'watermelon' exist in the hash? False