Not that this answers all your objections (it does not, caching might be a problem!) But sets are also a mere log(n) away https://hackage.haskell.org/package/

Not that this answers all your objections (it does not, caching might be a problem!) But sets are also a mere log(n) away https://hackage.haskell.org/package/containers-0.6.6/docs/Data-IntMap-Lazy.html https://hackage.haskell.org/package/containers-0.6.6/docs/Da...