Re: RFC 178 (v1) Lightweight Threads

Perl6 RFC Librarian wrote:
> =head2 Each thread gets its own copy of block-scoped lexicals upon
> execution of C<my>
> 
> Example 8
> 
>     #!/my/path/to/perl
>     foo();
>     Thread->new(\&foo);
> 
>     sub foo
>     {
>         my $a = 1;
>         print $a++;
>     }

[prints "11"]

This must be true otherwise $a is not a lexical variable. Threading
must not change the definition of a lexical.

The more interesting case is this:

    #!/my/path/to/perl
    sub foo_generator { my $a = shift; sub { print $a++ } }
    my $foo = foo_generator(1);
    $foo->();
    Thread->new($foo);

Is $a shared between threads or not? If $a is not shared, we've broken
lexicals.

IMHO the rule is not as simple as this RFC states. (Partly because of
confusion about "executing" my.)

Perhaps Thread->new should deep copy the code ref before executing
it? Deep copy lexicals but not globals? Deep copy anything that doesn't
already have a mutex lock?

I like the idea of having an RFC nail down the behavior of threads
though. It might be getting a bit into internals, but if the language
group doesn't decide the behavior, they'll have to live with whatever
internals cooks up.

- Ken