Loop variable in BEGIN block becomes a VMNull

[Leont]

The Problem

This code:

unit class Foo;

BEGIN {
    my $foo = 1;
    { $?CLASS.^add_method('foo', anon method () { dd $foo } ) }
} 

when run as

perl6 -I. -Mfoo -e'Foo.new.foo'

fails with

Cannot invoke this object (REPR: Null; VMNull)
  in sub  at /home/leont/Code/6pan/TestCase/lib/Foo.pm6 (Foo) line 4
  in method <anon> at /home/leont/Code/6pan/TestCase/lib/Foo.pm6 (Foo) line 5
  in block <unit> at -e line 1

unless one adds no precomplication to the class.

Environment

This is Rakudo version 2018.09 built on MoarVM version 2018.09

[AlexDaniel]

FWIW this is not a recent regression: https://gist.github.com/1d7ab9005e3c53fae9b91c5fcfdbe1f2

[Leont]

Some more investigation indicates it's not the loop per se that's essential in triggering this, but it needs a scope below the main BEGIN one

[zoffixznet]

but it needs a scope below the main BEGIN one

Doesn't look like even that is necessary. Just a core routine call inside the added method:

# Foo.pm6
unit class Foo;
BEGIN { $?CLASS.^add_method: 'foo', { join } }

# script.p6
use lib '.';
use Foo;
Foo.foo;

# OUTPUT: Cannot invoke this object (REPR: Null; VMNull)

And it happens only for core routine calls, not ones you define inside BEGIN.

And if you define a routine outside the begin and try to use it, you get a different error:

# Foo.pm6
unit class Foo;
sub join {}
BEGIN { $?CLASS.^add_method: 'foo', { join } }

# script.p6
use lib '.';
use Foo;
Foo.foo;

# OUTPUT: getlex: outer index out of range

[zoffixznet]

Found what could possibly be a workaround you could use: assign a sub you want to use to another symbol inside BEGIN and then use that symbol inside the code of the method:

unit class Foo;
BEGIN { my &sym2 = ⅆ $?CLASS.^add_method: 'foo', { sym2 42 } }

[Leont]

Found what could possibly be a workaround you could use: assign a sub you want to use to another symbol inside BEGIN and then use that symbol inside the code of the method:

That only work because &dd doesn't have to be kept alive. This for example will fail:

BEGIN { my &sym2 = sub ($arg) { dd $arg }; $?CLASS.^add_method: 'foo', { sym2 42 } }

[zoffixznet]

Yeah, I guess it's fairly limited; you have to extract &dd into that scope as well:

unit class Foo;
BEGIN {
    my &dd2 = ⅆ my &sym2 = sub ($arg) { dd2 $arg };
    $?CLASS.^add_method: 'foo', { sym2 42 } }

[niner]

This would probably be the place to start looking at: https://github.com/rakudo/rakudo/blob/master/src/Perl6/World.nqp#L2636
is_nested in that context means "BEGIN time EVAL"

[Leont]

Yeah, I guess it's fairly limited; you have to extract &dd into that scope as well:

That isn't going to help in my use-case, where I need to do this over many values.

[Leont]

This would probably be the place to start looking at: https://github.com/rakudo/rakudo/blob/master/src/Perl6/World.nqp#L2636
is_nested in that context means "BEGIN time EVAL"

That does indeed look like it deals with the outermost BEGIN scope and not further. Not that I'd know how to fix that.

[lizmat]

FWIW, a similar issue exists with:

use nqp;
BEGIN my $h := nqp::hash("a",42);

[vrurg]

Tested on 2019.07.1 – fails. HEAD passes.

Closing via MoarVM/MoarVM#1209 and 7e3d023