I’ve created a finger tree based Deque (double-ended queue).
Why? After all, there already is a elm-deque package.
This implementation provides amortized O(1) concatination/appending of deques, which the other package doesn’t. It also seems to be bit faster in all operations but push{Front/Back}, but this is based on benchmarks on my machine. Your results may vary.
https://package.elm-lang.org/packages/Skinney/elm-deque/latest/