I think this might be more interesting than actually useful, but the Berlin Functional Programming group just hosted a talk about using Souffle (a typed datalog) in Haskell: Leverage the Power of Logic Programming With Souffle-haskell - YouTube
There is plenty of stuff about API design & DSLs for datalog in there, although I suspect a lot of it probably relies on Haskell magic that might not be possible in Elm (I don’t really know enough about Haskell though).