The billstclair/elm-id-search package allows quick search for substrings of record identification strings.
You can use it to index identifying strings for any record type. For example, you could index User records by their userid to offer completions while typing "@foo" for a Twitter interface.
There's a live example, showing completion of lists of popular boy and girl baby names at https://billstclair.github.io/elm-id-search
An elm repl example:
$ elm repl
> import IdSearch exposing (..)
> ids = ["avh4","brianhicks","czaplic","eeue56","elmlang","lukewestby","mdgriffith","noredink","rtfeldman","terezka","zwilias"]
> table = makeTable 3 List.singleton |> insertList ids
{ dictCount = 3, dicts = [Dict ... ]
, getIdentifiers = <function> }
: Table String
> lookup "c" table
["czaplic"] : List String
> lookup "li" table
["zwilias","czaplic"] : List String
> lookup "hicks" table
["brianhicks"] : List String
> lookup "hicke" table
[] : List String