Simon, I was looking at the bytecode compiler to understand what's needed to support unboxed tuples. It seems like if we generate bytecode after unarise it should be very easy to support unboxed tuples, because after unarise we don't have any unboxed tuple binders (all binders have UnaryType). So only places we see unboxed tuples are: - Return positions. We just push contents of the tuple to the stack. - Case alternatives. The case expression in this case has to have this form: case e1 of (# bndr1, bndr2, ..., bndrN #) -> RHS All binders will have unary types again. We just bind ids in the environment to their stack locations and compile RHS. I think that's it. We also get unboxed sums support for free when we do this after unarise. What do you think about compiling to bytecode from STG? Have you considered that before? Would that be a problem for GHCi's debugger or any other features?