Why does `will end` behave differently than `will leave` in program-level scope? - raku

Question

I thought that in the top level of a program that will end and will leave would behave the same way, since there is only one big outer scope to exit/leave from. I thought that either one would be a good way to check a variable's final value.
But with will end it acts like the variable has never been initialized:
my $foo will end { put "Final value for \$foo is '$_'"} = 'bar';
put "\$foo is now '$foo'";
$foo ~= ' baz';
OUTPUT
$foo is now 'bar'
Use of uninitialized value $_ of type Any in string context.
Methods .^name, .perl, .gist, or .say can be used to stringify it to something meaningful.
in block at phaser_end.p6 line 1
Final value for $foo is ''
However, simply changing will end to will leave does what I would expect from either one:
my $foo will leave { put "Final value for \$foo is '$_'"} = 'bar';
put "\$foo is now '$foo'";
$foo ~= ' baz';
OUTPUT
$foo is now 'bar'
Final value for $foo is 'bar baz'
Why is there a difference in behavior here?
I am using Rakudo-Star 2017.07.
UPDATE
To get the effect that I'm expecting with will end, I have to use a separate END block:
END block:
my $foo = 'bar';
END { put "Final value for \$foo is '$foo'"};
put "\$foo is now '$foo'";
$foo ~= ' baz';
I guess the real question boils down to why does the END block behave differently than the will end block.
will end block:
my $foo will end { put "Final value for \$foo is '$_'"} = 'bar';
put "\$foo is now '$foo'";
$foo ~= ' baz';

Answer 1

Rewritten
Why does will end behave differently than will leave
It looks like will end has a bug similar to this old and now resolved bug for will begin.
Other than that, everything works as I would expect:
my $leave will leave { say ['leave:', $_, OUTERS::<$leave>] } = 'leave';
my $end will end { say ['end:', $_, OUTERS::<$end>] } = 'end';
my $begin will begin { say ['begin:', $_, OUTERS::<$begin>] } = 'begin';
END { say ['END:', $_, OUTERS::<$end>, $end] }
$_ = 999;
displays
[begin: (Any) (Any)]
[leave: leave leave]
[END: 999 end end]
[end: 999 end]
Is the scope for the block with will end different than the scope of the block with will leave?
They have the same outer lexical scope that corresponds to the UNIT scope.
They run in different dynamic scopes. Leave blocks run as part of leaving the enclosing block. End blocks run after the compiler has completely finished with your code and is cleaning up:
sub MAIN(#ARGS) {
...
# UNIT scope -- your code -- is about to be created and compiled:
$comp.command_line ...
# UNIT scope no longer exists. We compiled it, ran it, and left it.
...
# do all the necessary actions at the end, if any
if nqp::gethllsym('perl6', '&THE_END') -> $THE_END {
$THE_END()
}
}
why does the END block behave differently than the will end block?
Because you used $foo in one and $_ in the other.