[kdkeyser]

Haxl is actually an example of "the more impressive and more useful" approach: it is an abstraction embedded within a Haskell library, that allows you to express your "intent" in simple, standard Haskell code, decoupled from the way this intent will be reached (i.e. the execution is determined/optimized by the library)

Haskell allows this to be done in a way that makes if very hard for the user to break this abstraction (the type system will prevent you from doing this), while still allowing the full language power of Haskell to be used.