Unpacking single-field, single-strict-constructor GADTs and existentials

[David Feuer]

Unboxing, per se, is not required; only newtype optimization. I
believe Ed would probably have mentioned something when I discussed
the issue with him if he'd already had an idea for hacking around it.
Instead, he said he wanted the feature too.

On Tue, May 24, 2016 at 6:03 PM, Carter Schonwald
<carter.schonwald at gmail.com> wrote:
> Phrased differently: there's a subclass of existential data types which have
> a well behaved unboxed memory layout?
>
> @ David : have you tried simulating this in userland using eds structs /
> structures lib?
>
> On Tuesday, May 24, 2016, David Feuer <david.feuer at gmail.com> wrote:
>>
>> I should mention that while this does not require UNPACKing sum types (or
>> any of the difficult trade-offs that involves), it lets programmers
>> accomplish such unpacking by hand under sufficiently general conditions to
>> be quite useful in practice. As long as the set of types involved is closed,
>> it'll do.
>>
>> David Feuer <david.feuer at gmail.com> writes:
>>
>> > Given
>> >
>> > data Big a = B1 !(Small1 a) | B2 !(Small2 a) | B3 !(Small3 a), where the
>> > Small types are (possibly recursive) sums, it's generally possible to
>> > express that as something like
>> >
>> > data Selector = One | Two | Three
>> > data Big a = forall (x :: Selector) .
>> >    Big !(BigG x a)
>> > data BigG x a where
>> >   GB1a :: some -> fields -> BigG 'One a
>> >   GB1b :: fields -> BigG 'One a
>> >   GB2a :: whatever -> BigG 'Two a
>> >   GB3a :: yeah -> BigG 'Three a
>> >
>> > Making one big GADT from all the constructors of the "small" types, and
>> > then wrapping it up in an existential. That's what I meant about
>> > "unpacking". But for efficiency purposes, that wrapper needs the newtype
>> > optimization.
>>
>> Yes, but you'd need to unbox a sum in this case, no? I think this is the
>> first issue that you need to solve before you can talk about dealing
>> with the polymorphism issue (although hopefully Ömer will make progress
>> on this for 8.2).
>>
>> Cheers,
>>
>> - Ben