[perl #131783] [LTA] :delete holes in Arrays get turned to Mus when coercing to List or Slip

[Zoffix Znet via RT]

On Sat, 22 Jul 2017 18:48:38 -0700, c...@zoffix.com wrote:
> The current behaviour kinda makes sense when you squint at it, but
> today we had a user[^1]who was surprised by it, so I'm filing it as a
> ticket, if maybe there's some Better Way this can be done with.
> 
> When you :delete an element from the Array, you get nqp::null stuffed
> into the position, which gets converted to `is default` value when you
> look it up again:
> 
> <Zoffix__> m: use nqp; my @a is default(42) = <a b c>; @a[1]:delete;
> dd @a
> <camelia> rakudo-moar b14721: OUTPUT: «Array @a = ["a", 42, "c"]␤»
> 
> However, if you now convert that Array to a Slip or a List, the hole
> is left as a bare Mu and doesn't get turned into a default value,
> which is lost:
> 
> <Zoffix__> m: use nqp; my @a is default(42) = <a b c>; @a[1]:delete;
> dd @a.Slip
> <camelia> rakudo-moar b14721: OUTPUT: «slip("a", Mu, "c")␤»
> 
> <Zoffix__> m: use nqp; my @a is default(42) = <a b c>; @a[1]:delete;
> dd @a.List
> <camelia> rakudo-moar b14721: OUTPUT: «("a", Mu, "c")␤»
> 
> So it makes sense that without `is default` you don't get an `is
> default` value, but at the same time, Mu is not a great value to deal
> with...
> 
> [1] https://irclog.perlgeek.de/perl6/2017-07-22#i_14908846


And actually both .flat and .Seq do get `is default` value instead of Mu, so at 
the very least there's some inconsistency here:

    <Zoffix__> m: use nqp; my @a is default(42) = <a b c>; @a[1]:delete; dd 
@a.flat
    <camelia> rakudo-moar b14721: OUTPUT: «("a", 42, "c").Seq␤»
    <Zoffix__> m: use nqp; my @a is default(42) = <a b c>; @a[1]:delete; dd 
@a.Seq
    <camelia> rakudo-moar b14721: OUTPUT: «("a", 42, "c").Seq␤»