Containers - Data.Map

Monoid instance

Map k v provides a Monoid instance with the following semantics:

• mempty is the empty Map, i.e. the same as Map.empty
• m1 <> m2 is the left-biased union of m1 and m2, i.e. if any key is present both in m1 and m2, then the value from m1 is picked for m1 <> m2. This operation is also available outside the Monoid instance as Map.union.

Constructing

We can create a Map from a list of tuples like this:

A Map can also be constructed with a single value:

There is also the empty function.

Data.Map also supports typical set operations such as union, difference and intersection.

Checking If Empty

We use the null function to check if a given Map is empty:

Finding Values

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:

notMember is similar:

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

Inserting Elements

Inserting elements is simple:

Deleting Elements

Importing the Module

The Data.Map module in the containers package provides a Map structure that has both strict and lazy implementations.

When using Data.Map, one usually imports it qualified to avoid clashes with functions already defined in Prelude:

Map.empty -- give me an empty Map