Funny you should make the absurd claim that Avail has anything to do with Haskell, then link to a page filled with things that Haskell will never be able to do: 1) maps and sets whose equality is defined only by membership, not by order. 2) semi-heterogenous tuple types (the leading types are heterogenous, and all past a particular index are homogenous, while allowing a range of tuple lengths). 3) polymorphic behavior. Haskell supports polymorphism if all your definitions occur inside the same compilation unit... yeah, all the extensibility of a switch statement with a different syntax. Functional type systems and open/closed polymorphism are inherently incompatible.
I'm sure there are many other features in Avail that Haskell could never support, even in theory. A usable exception mechanism comes to mind.