This is a replacement package for Dict from elm/core. The API is identical, except needing to use equals instead of == for comparing two dictionaries.

The suggested way to use this package is by replacing import Dict with import FastDict as Dict in your project.

The main differences between Dict and FastDict are:

1. This is not a built-in type, so you can't use == with it. If you do you may get false negatives (dictionaries which are equal but that == considers different) - use FastDict.equals to check for equality;
2. size is O(1) instead of O(n) (i.e.: it runs in constant time instead of scanning the whole dictionary);
3. union automatically merges the smaller dictionary into the bigger (without changing the result), making it faster;
4. intersect is O(m + n) instead of O(m log n) and in practice is MUCH faster for small intersections (usually ranging from 2x faster to 100x faster);
5. equals is sometimes faster: it's O(1) if the size is different, and O(index of first different value) otherwise instead of O(m + n);
6. getMinKey/getMin/popMin/getMaxKey/getMax/popMax functions are available, with cost O(log n),
7. stoppableFoldl/stoppableFoldr/restructure functions are available.

• You use intersect, union or size a lot;
• you have big dictionaries;
• you need a fast getMin/getMax function;
• you need stoppableFolds or restructure.

• You need to interact with code that expects elm/core dictionaries a lot;
• you have tiny dictionaries;
• you have a lot of existing code that would need to be checked for uses of ==.

import FastDict as Dict exposing (Dict)

type alias Priority =
    Int

type alias Job =
    String

queue : Dict Priority Job
queue =
    Dict.fromList
        [ (3, "Shave the yak")
        , (5, "Reticulate splines")
        , (1, "Feed the gremlins")
        ]

{-| Returns the most important item

    mostImportant queue
    --> Just (1, "Feed the gremlins")

-}

mostImportant : Dict Priority Job -> Maybe (Priority, Job)
mostImportant =
    Dict.getMin