Strict unlifted types

[David Feuer]

Sometimes there's an awkward expressive tension between saying that a
type is *unboxed* and saying that it is *strict* in a field. For
example, I can write

data SMaybe a = SNothing | SJust !a

or I can write

type Maybe# a = (# (# #) | a #)

but there doesn't seem to be a way to simultaneously get both unboxed
and strict. I'm wondering if it might make sense to add a type to
serve the purpose

Strict# :: Type -> TYPE 'UnliftedRep

mkStrict# :: a -> Strict# a  -- force the argument
getStrict# :: Strict# a -> a  -- do nothing, but know the result is in WHNF

Then one could write, for example,

type SMaybe# a = (# (# #) | Strict# a #)

at which point SMaybe is *precisely* isomorphic to the alternative
representation

data SMaybe' a = SMaybe' (SMaybe# a)

David