Would it be possible to add unsafe primops to add Array# and SmallArray# entries to an ArrayArray#? The fact that the ArrayArray# entries are all directly unlifted avoiding a level of indirection for the containing structure is amazing, but I can only currently use it if my leaf level data can be 100% unboxed and distributed among ByteArray#s. It'd be nice to be able to have the ability to put SmallArray# a stuff down at the leaves to hold lifted contents. I accept fully that if I name the wrong type when I go to access one of the fields it'll lie to me, but I suppose it'd do that if i tried to use one of the members that held a nested ArrayArray# as a ByteArray# anyways, so it isn't like there is a safety story preventing this. I've been hunting for ways to try to kill the indirection problems I get with Haskell and mutable structures, and I could shoehorn a number of them into ArrayArrays if this worked. Right now I'm stuck paying for 2 or 3 levels of unnecessary indirection compared to c/java and this could reduce that pain to just 1 level of unnecessary indirection. -Edward