In Haskell, a hash (also called a map) is a collection of key-value pairs where each key is unique. You can access the value of a key in a hash using the Map.lookup function.
Here's an example hash in Haskell:
import qualified Data.Map as Map
myHash :: Map.Map String Int
myHash = Map.fromList [("apple", 1), ("banana", 2), ("orange", 3)]
This hash has keys as strings and values as integers.
To get the value of a key, use the Map.lookup function:
Map.lookup "apple" myHash -- returns Just 1
Map.lookup "pear" myHash -- returns Nothing
The Map.lookup function returns a Maybe value. If the key is present in the hash, it returns Just the corresponding value. Otherwise, it returns Nothing.
You can also use the Map.! operator to get the value of a key. This operator throws an error if the key is not present in the hash:
myHash Map.! "apple" -- returns 1
myHash Map.! "pear" -- throws an exception: "key not found"
It's generally safer to use Map.lookup, since it returns a Maybe value that you can handle gracefully if the key is not present in the hash.