On 08/09/2015 13:10, Simon Peyton Jones wrote:
| Without any unlifted kind, we need | - ArrayArray# | - a set of new/read/write primops for every element type, | either built-in or made from unsafeCoerce# | | With the unlifted kind, we would need | - ArrayArray# | - one set of new/read/write primops | | With levity polymorphism, we would need | - none of this, Array# can be used
I don't think levity polymorphism will work here. The code for a function needs to know whether an intermediate value of type 'a' is strict or not. It HAS to choose (unless we compile two versions of every function). So I don't see how to be polymorphic over a type variable that can range over both lifted and unlifted types.
The only reason that 'error' is levity-polymorphic over both lifted and unlifted types is that it never returns! error :: forall (a :: AnyKind). String -> a the code for error never manipulates a value of type 'a', so all is well. But it's an incredibly special case.
I think there's a bit of confusion here, Ed's email a bit earlier described the proposal for the third option above: https://mail.haskell.org/pipermail/ghc-devs/2015-September/009867.html For generalising these primops it would be fine, there are no thunks being built. Cheers Simon