FastDict

A dictionary mapping unique keys to values. The keys can be any comparable type. This includes Int, Float, Time, Char, String, and tuples or lists of comparable types.

Insert, remove, and query operations all take O(log n) time.

Dictionaries

type alias Dict k v =

Dict k v

A dictionary of keys and values. So a Dict String User is a dictionary that lets you look up a String (such as user names) and find the associated User.

import FastDict as Dict exposing (Dict)


users : Dict String User
users =
    Dict.fromList
        [ ( "Alice", User "Alice" 28 1.65 )
        , ( "Bob", User "Bob" 19 1.82 )
        , ( "Chuck", User "Chuck" 33 1.75 )
        ]


type alias User =
    { name : String
    , age : Int
    , height : Float
    }

Build

empty : Dict k v

Create an empty dictionary.

singleton : comparable -> v -> Dict comparable v

Create a dictionary with one key-value pair.

insert : comparable -> v -> Dict comparable v -> Dict comparable v

Insert a key-value pair into a dictionary. Replaces value when there is a collision.

update :

comparable

-> (Maybe v -> Maybe v)

-> Dict comparable v

Update the value of a dictionary for a specific key with a given function.

remove : comparable -> Dict comparable v -> Dict comparable v

Remove a key-value pair from a dictionary. If the key is not found, no changes are made.

Query

isEmpty : Dict k v -> Bool

Determine if a dictionary is empty.

isEmpty empty
--> True

member : comparable -> Dict comparable v -> Bool

Determine if a key is in a dictionary.

get : comparable -> Dict comparable v -> Maybe v

Get the value associated with a key. If the key is not found, return Nothing. This is useful when you are not sure if a key will be in the dictionary.

animals : Dict String String
animals =
    fromList [ ("Tom", "Cat"), ("Jerry", "Mouse") ]


get "Tom"   animals
--> Just "Cat"


get "Jerry" animals
--> Just "Mouse"


get "Spike" animals
--> Nothing

size : Dict k v -> Int

Determine the number of key-value pairs in the dictionary.

equals : Dict comparable v -> Dict comparable v -> Bool

Determine if two dictionaries are equal. This is needed because the structure could be different depending on insertion order.

Min / Max

getMinKey : Dict k v -> Maybe k

Gets the smallest key in the dictionary.

[ ( 1, 'z' ), ( 2, 'a' ) ]
    |> fromList
    |> getMinKey
--> Just 1




empty
    |> getMinKey
--> Nothing

getMin : Dict k v -> Maybe ( k, v )

Gets the key-value pair with the smallest key.

[ ( 1, 'z' ), ( 2, 'a' ) ]
    |> fromList
    |> getMin
--> Just ( 1, 'z' )




empty
    |> getMin
--> Nothing

getMaxKey : Dict k v -> Maybe k

Gets the biggest key in the dictionary.

[ ( 1, 'z' ), ( 2, 'a' ) ]
    |> fromList
    |> getMaxKey
--> Just 2




empty
    |> getMaxKey
--> Nothing

getMax : Dict k v -> Maybe ( k, v )

Gets the key-value pair with the biggest key.

[ ( 1, 'z' ), ( 2, 'a' ) ]
    |> fromList
    |> getMax
--> Just ( 2, 'a' )




empty
    |> getMax
--> Nothing

popMin :

Dict comparable v

-> Maybe ( ( comparable, v ), Dict comparable v )

Removes the key-value pair with the smallest key from the dictionary, and returns it.

[ ( 1, 'z' ), ( 2, 'a' ) ]
    |> fromList
    |> popMin
--> Just ( ( 1, 'z' ), fromList [ ( 2, 'a' ) ] )




empty
    |> popMin
--> Nothing

popMax :

Dict comparable v

-> Maybe ( ( comparable, v ), Dict comparable v )

Removes the key-value pair with the biggest key from the dictionary, and returns it.

[ ( 1, 'z' ), ( 2, 'a' ) ]
    |> fromList
    |> popMax
--> Just ( ( 2, 'a' ), fromList [ ( 1, 'z' ) ] )




empty
    |> popMax
--> Nothing

Lists

keys : Dict k v -> List k

Get all of the keys in a dictionary, sorted from lowest to highest.

keys (fromList [ ( 0, "Alice" ), ( 1, "Bob" ) ])
--> [ 0, 1 ]

values : Dict k v -> List v

Get all of the values in a dictionary, in the order of their keys.

values (fromList [ ( 0, "Alice" ), ( 1, "Bob" ) ])
--> [ "Alice", "Bob" ]

toList : Dict k v -> List ( k, v )

Convert a dictionary into an association list of key-value pairs, sorted by keys.

fromList : List ( comparable, v ) -> Dict comparable v

Convert an association list into a dictionary.

Transform

map : (k -> a -> b) -> Dict k a -> Dict k b

Apply a function to all values in a dictionary.

foldl : (k -> v -> b -> b) -> b -> Dict k v -> b

Fold over the key-value pairs in a dictionary from lowest key to highest key.

getAges : Dict String Int -> List Int
getAges usersDict =
    FastDict.foldl addAge [] usersDict


addAge : String -> Int -> List Int -> List Int
addAge _ age ages =
    age :: ages


users : Dict String Int
users =
    FastDict.fromList
        [ ( "Abe", 28 )
        , ( "Beatrix", 19 )
        , ( "Charlotte", 33 )
        ]


-- Note that the _fold_ is from lowest to highest,
-- but because we're adding items to the beginning of the list
-- the result will be from highest to lowest.


getAges users
--> [ 33, 19, 28 ]

foldr : (k -> v -> b -> b) -> b -> Dict k v -> b

Fold over the key-value pairs in a dictionary from highest key to lowest key.

getAges : Dict String Int -> List Int
getAges usersDict =
    FastDict.foldr addAge [] usersDict


addAge : String -> Int -> List Int -> List Int
addAge _ age ages =
    age :: ages


users : Dict String Int
users =
    FastDict.fromList
        [ ( "Abe", 28 )
        , ( "Beatrix", 19 )
        , ( "Charlotte", 33 )
        ]


-- Note that the _fold_ is from highest to lowest,
-- but because we're adding items to the beginning of the list
-- the result will be from lowest to highest.


getAges users
--> [ 28, 19, 33 ]

filter : (comparable -> v -> Bool) -> Dict comparable v -> Dict comparable v

Keep only the key-value pairs that pass the given test.

partition :

(comparable -> v -> Bool)

-> Dict comparable v

-> ( Dict comparable v, Dict comparable v )

Partition a dictionary according to some test. The first dictionary contains all key-value pairs which passed the test, and the second contains the pairs that did not.

Combine

union : Dict comparable v -> Dict comparable v -> Dict comparable v

Combine two dictionaries. If there is a collision, preference is given to the first dictionary.

intersect : Dict comparable v -> Dict comparable v -> Dict comparable v

Keep a key-value pair when its key appears in the second dictionary. Preference is given to values in the first dictionary.

diff : Dict comparable a -> Dict comparable b -> Dict comparable a

Keep a key-value pair when its key does not appear in the second dictionary.

merge :

(comparable -> a -> result -> result)

-> (comparable -> a -> b -> result -> result)

-> (comparable -> b -> result -> result)

-> Dict comparable a

-> Dict comparable b

-> result

The most general way of combining two dictionaries. You provide three accumulators for when a given key appears:

Only in the left dictionary.
In both dictionaries.
Only in the right dictionary.

You then traverse all the keys from lowest to highest, building up whatever you want.

Interoperability

toCoreDict : Dict comparable v -> Dict comparable v

Convert the dictionary into an equivalent one from elm/core.

fromCoreDict : Dict comparable v -> Dict comparable v

Convert the dictionary from an equivalent one from elm/core.

Advanced functions

type Step a

= Continue a

| Stop a

A custom type used for stoppable folds.

stoppableFoldl : (k -> v -> acc -> Step acc) -> acc -> Dict k v -> acc

A foldl that can stop early instead of traversing the whole dictionary.

stoppableFoldl
    (\k v acc ->
        if k >= 10 then
            Stop acc
        else
            Continue (v + acc)
    )
    0
    (fromList <| List.indexedMap Tuple.pair <| List.range 1 10000)
--> 55

stoppableFoldr : (k -> v -> acc -> Step acc) -> acc -> Dict k v -> acc

A foldr that can stop early instead of traversing the whole dictionary.

stoppableFoldr
    (\k v acc ->
        if k <= 9990 then
            Stop acc
        else
            Continue (v + acc)
    )
    0
    (fromList <| List.indexedMap Tuple.pair <| List.range 1 10000)
--> 89964

restructure :

acc

-> ( { key : key

, value : value

, left : () -> acc

, right : () -> acc

}

-> acc

)

-> Dict key value

-> acc

This allows you to take advantage of the tree structure of the dictionary to do some operations more efficiently.

Calling left will give the result of calling restructure on the left subtree (lower keys), right on the right one (higher keys).

If this is confusing you probably don't need this function!

any dict =
    -- Notice how if `value` is `True` we don't call `left` nor `right`,
    -- and if `value` is `False` but `left ()` is `True` we don't call right.
    restructure False (\{ value, left, right } -> value || left () || right ())

miniBill/elm-fast-dict/1.2.5