Restricted sums in BoxedRep

[Spiwack, Arnaud]

I may have misunderstood, but my understanding is the following:

- Since a is a boxed type, it can never be the null pointer
- So I can use a null pointer unambiguously

Let's call this null-pointer expanded type, `Nullable# a`, it is now a
different sort than `a`, since it can have the null pointer in it. Is that
still what you wish for? The risk being that combining such a `Nullable# a`
with another data structure may very well require an additional boxing,
which is what, I believe, you were trying to avoid.

/Arnaud

On Tue, Oct 13, 2020 at 11:47 PM David Feuer <david.feuer at gmail.com> wrote:

> Null pointers are widely known to be a lousy language feature in general,
> but there are certain situations where they're *really* useful for compact
> representation. For example, we define
>
>     newtype TMVar a = TMVar (TVar (Maybe a))
>
> We don't, however, actually use the fact that (Maybe a) is lifted. So we
> could represent this much more efficiently using something like
>
>     newtype TMVar a = TMVar (TVar a)
>
> where Nothing is represented by a distinguished "null" pointer.
>
> While it's possible to implement this sort of thing in user code (with
> lots of fuss and care), it's not very nice at all. What I'd really like to
> be able to do is represent certain kinds of sums like this natively.
>
> Now that we're getting BoxedRep, I think we can probably make it happen.
> The trick is to add a special Levity constructor representing sums of
> particular shapes. Specifically, we can represent a type like this if it is
> a possibly-nested sum which, when flattened into a single sum, consists of
> some number of nullary tuples and at most one Lifted or Unlifted type.
> Then we can have (inline) primops to convert between the BoxedRep and the
> sum-of-sums representations.
>
> Anyone have thoughts on details for what the Levity constructor arguments
> might look like?
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