# Module DA.Map¶

Note: This is only supported in DAML-LF 1.11 or later.

This module exports the generic map type Map k v and associated functions. This module should be imported qualified, for example:

import DA.Map (Map)
import DA.Map qualified as M


This will give access to the Map type, and the various operations as M.lookup, M.insert, M.fromList, etc.

Map k v internally uses the built-in order for the type k. This means that keys that contain functions are not comparable and will result in runtime errors. To prevent this, the Ord k instance is required for most map operations. It is recommended to only use Map k v for key types that have an Ord k instance that is derived automatically using deriving:

data K = ...
deriving (Eq, Ord)


This includes all built-in types that aren’t function types, such as Int, Text, Bool, (a, b) assuming a and b have default Ord instances, Optional t and [t] assuming t has a default Ord instance, Map k v assuming k and v have default Ord instances, and Set k assuming k has a default Ord instance.

## Functions¶

fromList

: Ord k => [(k, v)] -> Map k v

Create a map from a list of key/value pairs.

fromListWith

: Ord k => (v -> v -> v) -> [(k, v)] -> Map k v

Create a map from a list of key/value pairs with a combining function. Examples:

>>> fromListWith (++) [("A", [1]), ("A", [2]), ("B", [2]), ("B", [1]), ("A", [3])]
fromList [("A", [1, 2, 3]), ("B", [2, 1])]
>>> fromListWith (++) [] == (empty : Map Text [Int])
True

keys

: Map k v -> [k]

Get the list of keys in the map. Keys are sorted according to the built-in order for the type k, which matches the Ord k instance when using deriving Ord.

>>> keys (fromList [("A", 1), ("C", 3), ("B", 2)])
["A", "B", "C"]

values

: Map k v -> [v]

Get the list of values in the map. These will be in the same order as their respective keys from M.keys.

>>> values (fromList [("A", 1), ("B", 2)])
[1, 2]

toList

: Map k v -> [(k, v)]

Convert the map to a list of key/value pairs. These will be ordered by key, as in M.keys.

empty

: Map k v

The empty map.

size

: Map k v -> Int

Number of elements in the map.

null

: Map k v -> Bool

Is the map empty?

lookup

: Ord k => k -> Map k v -> Optional v

Lookup the value at a key in the map.

member

: Ord k => k -> Map k v -> Bool

Is the key a member of the map?

filter

: Ord k => (v -> Bool) -> Map k v -> Map k v

Filter the Map using a predicate: keep only the entries where the value satisfies the predicate.

filterWithKey

: Ord k => (k -> v -> Bool) -> Map k v -> Map k v

Filter the Map using a predicate: keep only the entries which satisfy the predicate.

delete

: Ord k => k -> Map k v -> Map k v

Delete a key and its value from the map. When the key is not a member of the map, the original map is returned.

insert

: Ord k => k -> v -> Map k v -> Map k v

Insert a new key/value pair in the map. If the key is already present in the map, the associated value is replaced with the supplied value.

alter

: Ord k => (Optional v -> Optional v) -> k -> Map k v -> Map k v

Update the value in m at k with f, inserting or deleting as required. f will be called with either the value at k, or None if absent; f can return Some with a new value to be inserted in m (replacing the old value if there was one), or None to remove any k association m may have.

Some implications of this behavior:

alter identity k = identity alter g k . alter f k = alter (g . f) k alter (_ -> Some v) k = insert k v alter (_ -> None) = delete

union

: Ord k => Map k v -> Map k v -> Map k v

The union of two maps, preferring the first map when equal keys are encountered.

merge

: Ord k => (k -> a -> Optional c) -> (k -> b -> Optional c) -> (k -> a -> b -> Optional c) -> Map k a -> Map k b -> Map k c

Combine two maps, using separate functions based on whether a key appears only in the first map, only in the second map, or appears in both maps.