Implicit reboxing of unboxed tuple in let-patterns

[Richard Eisenberg]

> On Sep 3, 2020, at 1:51 PM, Iavor Diatchki <iavor.diatchki at gmail.com> wrote:
> 
> How about the following rule:  unlifted patterns are always strict (i.e., you desugar them as if they had an explicit  `!`).   A pattern is "unlifted" if the type it examines is unlifted.   Seems simple enough and, I think, it would do what most folks would expect.  

I don't think it's this simple. For example:

> data X = MkX Int#
>
> a = let MkX 3# = undefined in ()
> b = let MkX z = undefined in ()
> c = let MkX _ = undefined in ()
> d = let MkX {} = undefined in ()
> e = let _ :: X = undefined in ()

Which of these diverge? e definitely converges, as X is lifted. b definitely diverges, because it binds z, a variable of an unlifted type, to a component of a diverging computation.

In GHC today, all the cases other than b converge.

Iavor suggests that a should diverge: 3# is a pattern of an unlifted type. What about c? What about d? Very unclear to me.

Note that banging the pattern nested inside the MkX does not change the behavior (in GHC today) for any of the cases where this makes sense to do so.

Richard

> 
> I guess a more explicit option would be to make it an error to use a lazy pattern on an unlifted type, and require programmers to manually add the `!` but I am not sure that gains much, and is more work in the compiler.
> 
> -Iavor
> 
> On Thu, Sep 3, 2020 at 10:38 AM Richard Eisenberg <rae at richarde.dev <mailto:rae at richarde.dev>> wrote:
> 
> 
>> On Sep 3, 2020, at 12:10 PM, Spiwack, Arnaud <arnaud.spiwack at tweag.io <mailto:arnaud.spiwack at tweag.io>> wrote:
>> 
>> This is all a tad tricky, I must say.
> 
> ... which is one of the reasons I originally wanted one simple rule. I'm not now saying I was in the right, but it is an attractive resting point for this discussion.
> 
> To be clear, I don't think there's going to be any concrete action here without a proposal, so perhaps once this thread finds a resting point different than the status quo, someone will have to write it up.
> 
> Richard
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