Haskell Language Containers - Data.Map Finding Values


Example

There are many querying operations on maps.

member :: Ord k => k -> Map k a -> Bool yields True if the key of type k is in Map k a:

> Map.member "Alex" $ Map.singleton "Alex" 31
True
> Map.member "Jenny" $ Map.empty
False

notMember is similar:

> Map.notMember "Alex" $ Map.singleton "Alex" 31
False
> Map.notMember "Jenny" $ Map.empty
True

You can also use findWithDefault :: Ord k => a -> k -> Map k a -> a to yield a default value if the key isn't present:

Map.findWithDefault 'x' 1 (fromList [(5,'a'), (3,'b')]) == 'x'
Map.findWithDefault 'x' 5 (fromList [(5,'a'), (3,'b')]) == 'a'