Self-referring labels

Lizmat kindly allowed Label to expose its file and line-number. That is handy if we want to convey messages about the code itself, without having to worry about edits invalidating our line-numbers. The first use case that came to mind are lightweight singletons that are easy to find.

barcode: Nil;
qrcode: Nil;

say [barcode ~~ qrcode, barcode === qrcode, barcode =:= qrcode]; # [False False False]
put barcode; # barcode ../label-introspection.raku:16

This might be handy when sending messages through a Channel.

my $ch = Channel.new;
start {
    HERE: Nil;
    THERE: Nil;
    $ch.send(.item) for ((1, 2, HERE, THERE) xx ∞).flat;
}

.put for @$ch;
# OUTPUT: 1
#         2
#         HERE ../label-introspection.raku:24
#         THERE ../label-introspection.raku:25
#         …

If those signals end up in a Str unintended, we have a good chance to find the source of the error, even when we have to look at the sender-end of a Channel.

We can also create error messages that point to a different line then a stacktrace might.

sub may-return-nil { }

ENIL: my $var is default(Failure.new(ENIL)) = may-return-nil;
say $var;

EWHOOPSY: fail(EWHOOPSY);

CATCH {
    when X::AdHoc && .payload ~~ Label {
        put "WELP! I encountered {.name} in {.file}:{.line}" with .payload;
    }
}

POD doesn’t allow us to do compile time interpolation (yet). Since it is made up of Arrays, we can cheat.

DOCLABEL: sub described {

}

=begin pod
Sub described is defined in L<PLACEHOLDER>.
=end pod

CHECK $=pod[0].contents[0].contents[1] = DOCLABEL.&{.file ~ ':' ~ .line};

say $=pod;

# OUTPUT: [Pod::Block::Named{:name("pod")}
#             Pod::Block::Para
#             Sub described is defined in
#             ../label-introspection.raku:31
#             .
#         ]

There are quite a few things hidden in CORE and I don’t like to use nqp::attr to get hold of them. A public interface is better then an accidental one. The former make way better idioms.