I looked at finger trees as a possible basis for the string representation in xi-editor, but ended up going with a simpler b-tree based approach. The better asymptotic bounds for doing manipulations at the ends are appealing in theory, but I never saw an actual problem with the O(log n) cost in practice, and it is possible to optimize the common append-only case a lot (I have a "builder" API but the current implementation is not as heavily optimized as it might be).

I _believe_ that the polymorphic recursive type, easily expressible in Haskell, cannot be expressed in Rust. You'd fake it by just using trees and having the shape as an invariant maintained by the library (just as the min and max child count constraint is maintained in a B-tree). I personally think that's fine, Rust wouldn't be a better language if its type system was made even more rich, but it's interesting to have examples so you know where the edges are. (there's also the possibility someone will find a way to encode it anyway)