Excerpts from Dan Doel's message of 2015-09-04 14:48:49 -0700:
I don't really understand what this example is showing. I don't think SPair is a legal declaration in any scenario.
- In current Haskell it's illegal; you can only put ! directly on fields - If !a :: Unlifted, then (,) (!a) is a kind error (same with Force a)
This is true. Perhaps it should be possible to define data types which are levity polymorphic, so SPair can kind as * -> * -> *, Unlifted -> Unlifted -> *, etc.
I don't think it interacts any differently than with unpacked/unboxed products today.
I meant like:
If T :: Unlifted, then am I allowed to do:
data U = MkU {-# UNPACK #-} T ...
and what are its semantics? If T is a sum, presumably it's related to the unpacked sums proposal from a couple days ago. Does stuff from this proposal make that proposal simpler? Should they reference things in one another?
Ah, this is a good question. I think you can just directly UNPACK unlifted types, without a strict bang pattern. I've added a note to the proposal.
Will there be optimizations that turn:
data E a b :: Unlifted where L :: a -> E a b R :: b -> E a b
into |# a , b #| (or whatever the agreed upon syntax is)? Presumably yes.
Yes, it should follow the same rules as https://ghc.haskell.org/trac/ghc/wiki/UnpackedSumTypes#Unpacking Edward