Are `Captures` value types or not?

[lizmat]

Capture instances are currently implemented as being a value type (basically since https://github.com/rakudo/rakudo/commit/c855a0f439 ).

However, since https://github.com/Raku/old-issue-tracker/issues/4360 it has made elements of it mutable (if they were created with containers).

If an element in a Capture is changed, it's associated .WHICH will also change. This violates the principle of immutability of value types.

Making Capture instances immutable, breaks spectests. Making the Capture class an object type, also breaks spectests.

[lizmat]

Propose to:

• keep Capture being a value type
• disallow mutations in the object (remove is raw from AT-POS / AT-KEY)
• adapt spectests accordingly

Ideally to be done with a language level check, but I think that will turn out to be pretty hard to do.

[jnthn]

The primary purpose of Capture in the language is to model an argument list, and arguments may be mutated by is rw parameters. For example, consider:

sub foo($p is rw) {
    $p++;
}

my $x = 1;
my \c = \($x);
foo(|c);
say c;

[lizmat]

@jnthn So then it shouldn't be a value type then?

[jnthn]

Correct, although it'd make sense for snapshot equality (eqv) to work.