Implicit reboxing of unboxed tuple in let-patterns

[Spiwack, Arnaud]

On Fri, Sep 4, 2020 at 5:47 PM Iavor Diatchki <iavor.diatchki at gmail.com>
wrote:

Yes
>
Interesting. Thanks.

Personally, I don’t really know how to resolve the tension between the
outer pattern saying: I *really* want to be lazy; and uniformity with the
unlifted-variable-binding case.

The point about b

b = let ~(MkX z) = undefined in ()

Is that it *cannot* do anything else than force the pattern. Because b is
an unlifted variable, it doesn’t contain a thunk. In other words, we can
read it as the pattern being lazy, but it being immediately forced. Pretty
much as in:

let (x,y) = undefined in x `seq` ()

My intuition would be that

let (x, (# #)) = (1, undefined) in ()

converges, while

let (x, (# #)) = (1, undefined) in x `seq` ()

diverges.

But I’m not exactly sure how to explain this rule succinctly. And I’m not
sure it would be quite as intuitive to anybody. For starters, it seems to
depart significantly from Iavor’s intuition.
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