Fixed with https://github.com/rakudo/rakudo/commit/c229022cb0 , tests needed
> On 12 Aug 2017, at 14:36, Peter du Marchie van Voorthuysen (via RT) > <perl6-bugs-follo...@perl.org> wrote: > > # New Ticket Created by Peter du Marchie van Voorthuysen > # Please include the string: [perl #131887] > # in the subject line of all future correspondence about this issue. > # <URL: https://rt.perl.org/Ticket/Display.html?id=131887 > > > > This is Rakudo version 2017.07 built on MoarVM version 2017.07 > implementing Perl 6.c. > > If the value of a Pair is a Scalar container, then the Pair can be > modified, e.g. > >> my $value = 0; > 0 >> my $pair = number => $value; > number => 0 >> $pair.value = 1; $pair; > number => 1 > > Method freeze make the value of the Pair read-only, by removing it from its > Scalar container, and returns the value. > >> $pair.freeze; > 1 >> $pair.value = 2; > Cannot modify an immutable Int (1) > in block <unit> at <unknown file> line 1 > > The problem is that freeze does more than that. It changes the object > identity as returned by WHICH as well: > >> $pair = number => $value; > number => 1 >> $pair.WHICH; > Pair|127791728 >> $pair.freeze; > 1 >> $pair.WHICH; > Pair|Str|number|Int|1 > > Now by itself having a 2-tuple that is identified by its two elements is a > nice feature (if it would be documented). But _changing_ the object > identity is not consistent with the behavior of other built-in Perl 6 > classes and actually breaks the implementation of some of these classes. > > For example, a SetHash represents a mutable set. The Set method returns a > _new_ object that is immutable: > >> $pair = number => $value; > number => 1 >> my $set = SetHash.new($pair); > SetHash.new(number => 1) >> my $set2 = $set.Set; > set(number => 1) >> $set.WHICH; > SetHash|136539408 >> $set2.WHICH; > Set|0EC3BFFD57719F5C6A3EE91A5EFAA5AEFE273964 > > But because freezing a Pair changes the identity of the _original_ object > it's possible to add a second instance of the _same_ Pair to the SetHash, > causing it to violate its contract: > >> $pair.freeze; > 1 >> $set{$pair} = True; > True >> my ($a, $b) = $set.keys; > (number => 1 number => 1) >> $a === $b; > True > > I think it's clear that changing the identity of the original object is not > correct. So I propose to remove the undocumented behavior of the freeze > method that it changes the object identity. > > Now I can imagine that at some implementation level there are benefits to > having a kind of Pair that is identified by its key _and_ value. I also > think it could be generally useful to have a class implementing a truly > immutable (2-)tuple that is identified by its elements. But that should be > a separate class and the Pair class should provide a method to create a > _new_ object of this class from a Pair object.
