This is in a similar space to reactive programming, but isn't reactive at its core. The best explanation of the monad we use developed is described in the paper here: http://community.haskell.org/~simonmar/papers/haxl-icfp14.pd...

Query sharding is at the data source layer, which Haxl doesn't delve into. It's up to each data source integration with Haxl to do the appropriate routing/etc.

Hope you find it useful!