> On 23 Jul 2017, at 03:48, Zoffix Znet (via RT) <perl6-bugs-follo...@perl.org> > wrote: > > # New Ticket Created by Zoffix Znet > # Please include the string: [perl #131783] > # in the subject line of all future correspondence about this issue. > # <URL: https://rt.perl.org/Ticket/Display.html?id=131783 > > > > 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
The problem is really caused by the fact that an Array has a descriptor, which contains the default value and type information, and a List does not. And since a Slip isa List, we lose the descriptor when Slipping an Array. The same issue appears with: $ 6 'my @a; @a[2] = 42; dd (|@a)[0] = 44; dd @a' Cannot modify an immutable Str (Nil) Which is because Slip is a List, it uses List.AT-POS, and that one doesn’t create a WHENCE with a container to be filled at a later time. Which then goes back to: what is the use case of Slipping an Array? When do you need that? Perhaps slipping an Array should produce a warning?
