On Sun, 13 Aug 2017 15:14:41 -0700, elizabeth wrote: > 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.
Test added with commit https://github.com/perl6/roast/commit/d7d42d6c37
