Archive for May, 2019

Whatever whenever does

May 31, 2019 1 comment

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> {
    || <?{
            nqp::getcomp('perl6').language_version eq '6.c'
          || $*WHENEVER_COUNT >= 0
    || <.typed_panic('X::Comp::WheneverOutOfScope')>
    { $*WHENEVER_COUNT++ }

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 := $<xblock>.ast;
        :op<call>, :name<&WHENEVER>, :node($/),
        $xblock[0], block_closure($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 $*IN that type will typically be whatever IO::Handle.lines returns.!LINES-ITERATOR($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 =;
    start {
        for callsame() {
            last if $channel.closed;
        LEAVE $channel.close unless $channel.closed;

Or if we want to be explicit:

use Concurrent::Channelify;

react {
    whenever signal(SIGINT) {
        say "Got signal";
    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.

Categories: Perl6

Nil shall warn or fail but not both

May 14, 2019 1 comment

As announced earlier I went to write a module to make Nil.list behave a little better. There are basicly two way Nil could be turned into a list. One should warn the same way as Nil.Str does and the other should end the program loudly. Doing both at the same time however does not make sense.

There are a few ways this could be done. One is augmenting Nil with a list method and have this method check a dynamic variable to pick the desired behaviour. That would be slow and might hurt if Nil.list is called in a loop. The other is by using a custom sub EXPORT and a given switch.

# lib/NoNilList/Warning.pm6
use NoNilList 'Warning';
# lib/NoNilList/Fatal.pm6
use NoNilList 'Fatal';
# lib/NoNilList.pm6

sub EXPORT($_?) {
    given $_ {
        when 'Warning' {
             # augment Nil with a warning .list
        when 'Fatal' {
             # augment Nil with a failing .list
        default {
            die 'Please use NoNilList::Warning or NoNilList::Fatal.';

    %() # Rakudo complains without this

Now use NoNilList; will yield a compile time error with a friedly hint how to avoid it.

I left the augmenting part out because it does not work. I thought I stepped on #2779 again but was corrected that this is acually a different bug. Jnthn++ fixed part of that new bug (Yes, Perl 6 bugs are so advanced they come in multiple parts.) and proposed the use of the MOP instead. That resulted in #2897. The tricky bit is that I have to delay augmentation of Nil to after the check on $_ because augment is a declarator and as such executed at compile time — in a module that can be months before the program starts to run. Both an augment in an EVAL string and the MOP route would lead there. I wanted to use this module as my debut on 6PAN. That will have to wait for another time.

If you find a bug please file it. It will lead to interresting discoveries for sure.

Categories: Perl6

MONKEY see no Nil

May 4, 2019 Leave a comment

In a for loop Nil is turned into a List with one Element that happens to be Any. This really buged me so I went to find out why. As it turns out the culprit is the very definition of Nil is Cool. To be able to turn any single value into a List Cool implements method list(). Which takes a single values and turns that value into a List with that one value. Nil indicates the absense of a value and turning it into a value doesn’t make sense. Luckily we can change that.


augment class Nil {
    method list() {
        note 'Trying to turn Nil into a list.';


sub niler() { Nil }

for niler() { say 'oi‽' }

We can’t just warn because that would show the wrong point in the stack trace. So we note (which also goes to $*ERR) and pull the last value out of the Backtrace.

Interestingly Failure throws both in .list and in .iterator. Nil implements push, append, unshift and prepend by immediatly die-ing. Adding more to nothing is deadly but turning nothing first into something vaguely undefined and then allowing to add more stuff to it is inconsistent at best. What leads me to believe that Nil.list as it is specced today is just an oversight.

At least I can now write a simple module to protect my code from surprising Nils.

Categories: Perl6