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Whatever whenever does

Jnthn answered the question why $*IN.lines blocks in a react block. What isn’t explained is what whenever actually does before it starts blocking.

react {
    whenever $*IN.lines { .say }
}

Looking at the syntax of a whenever block, we see that whenever takes a variable immediatly followed by a block. The only place where a structure like that can be defined is Grammar.nqp.

rule statement_control:sym<whenever> {
    <sym><.kok>
    [
    || <?{
            nqp::getcomp('perl6').language_version eq '6.c'
          || $*WHENEVER_COUNT >= 0
       }>
    || <.typed_panic('X::Comp::WheneverOutOfScope')>
    ]
    { $*WHENEVER_COUNT++ }
    <xblock($PBLOCK_REQUIRED_TOPIC)>
}

Here the grammar just checks a few things without actually generating any code. So we head to Actions.nqp.

method statement_control:sym<whenever>($/) {
    my $xblock := $<xblock>.ast;
    make QAST::Op.new(
        :op<call>, :name<&WHENEVER>, :node($/),
        $xblock[0], block_closure($xblock[1])
    );
}

The whenever block is converted to a call to sub WHENEVER which we find in Supply.pm6.

sub WHENEVER(Supply() $supply, &block) {

There we go. A whenever block takes its first argument of any type and calles .Supply on it, as long as Any is a parent of that type. In the case of $*IN that type will typically be whatever IO::Handle.lines returns.

Seq.new(self!LINES-ITERATOR($close))

To turn a Seq into a Supply Any.Supply calls self.list.Supply. Nowhere in this fairly long chain of method lookups (this can’t be fast) are there any threads to be found. If we want to fix this we need to sneak a Channel into $*IN.lines which does exactly that.

$*IN.^can('lines')[1].wrap(my method {
    my $channel = Channel.new;
    start {
        for callsame() {
            last if $channel.closed;
            $channel.send($_)
        }
        LEAVE $channel.close unless $channel.closed;
    }
    $channel
});

Or if we want to be explicit:

use Concurrent::Channelify;

react {
    whenever signal(SIGINT) {
        say "Got signal";
        exit;
    }
    whenever $*IN.lines⇒ {
        say "got line";
    }
}

We already use ⚛ to indicate atomic operations. Maybe using prefix:<∥> to indicate concurrency makes sense. Anyway, we went lucky once again that Rakudo is implemented (mostly) in Perl 6 so we can find out where we need to poke it whenever we want to change it.

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Categories: Perl6
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