Janiczek/elm-bidict

A dictionary data structure with a twist: it allows you to ask for the reverse mapping (from values to keys).

myMapping : BiDict String Int
myMapping =
    BiDict.empty
        |> BiDict.insert "A" 1
        |> BiDict.insert "B" 2
        |> BiDict.insert "C" 1
        |> BiDict.insert "D" 4

BiDict.getReverse 1 myMapping
--> Set.fromList ["A", "C"]

There is also MultiBiDict: it not only remembers the reverse mapping, it also allows you to have multiple values per key:

myMapping : MultiBiDict String Int
myMapping =
    MultiBiDict.empty
        |> MultiBiDict.insert "A" 1
        |> MultiBiDict.insert "B" 2
        |> MultiBiDict.insert "C" 3
        |> MultiBiDict.insert "A" 2

MultiBiDict.get "A" myMapping
--> Set.fromList [1, 2]

MultiBiDict.getReverse 2 myMapping
--> Set.fromList ["A", "B"]

There are both a Dict-using and an assoc-list-using variant for each of these. Thus, one needs comparable keys and values and the other can hold any types, but has slightly worse performance characteristics.