I feel a bit like the kid outside the sweet shop (candy store), or toy shop. Allowed to look, but not to touch:)

We (the designers) feel that way too! Unfortunately both the design and the implementation are currently unfunded volunteer efforts, sandwiched in between our day jobs (or, increasingly, our search for a day job). That translates to a development process far slower than any of us would prefer. :-(

I find all of that extremely readable. Obviously your copius comments and context help, but it still bodes well for the future.

Thank-you. We've worked incredibly hard on keeping Perl 6 Perlish; tried to make it even more Perlish, in fact. Of course, threshing True Perl™ out the myriad of possibilities also contributes to the slowness of development.

I see you using (what I take to be) the P6 equivalents of P5s $a and $b.

Not quite. $^a and $^b are "placeholder variables". Every block in Perl 6 is really an anonymous subroutine. Putting a placeholder in a block is one way of giving that anonymous subroutine a parameter. Putting two in a block, gives the block/subroutine two parameters. Etc. etc.

Apart from that they are presumable properly scoped removing the old global clash fears,

Indeed. Like all subroutine parameters, they are lexically scoped to the body of the subroutine. In this case, to the block itself.

are they limited to just those two?

No. Any subroutine can have any number of parameters. Hence, any block can have any number of placeholders.

if I need to look at more than two elements of the argument array at a time (eg. a moving averages calculation) can I get to them through $^c, $^d etc?

You can specify as many placeholders as you like. However, the reduce built-in will not compute a moving average for you. It takes the data list you provide and successively extracts as many elements as there are parameters to the reduction block. To get a moving average, or moving maximum, or moving whatever, we'd need a function that takes an N-argument subroutine and moves it over a window. For example:

sub moving (&func, *@data) {
    # Note: "arity" means how many params the sub takes
    my @results;
    for 0..@data-&block.arity -> $from {
        my $to = $from + &block.arity - 1;
        push @results, func(@data[$from..$to]);  
    }
    return @results;
}

@average_3   = moving { ($^x+$^y+$^z)/3 } @values;
@average_4   = moving { ($^w+$^x+$^y+$^z)/4 } @values;
@local_max_3 = moving { max $^x, $^y, $^z } @values;
# etc.
[download]

Will @args (and @values) be implemented as a list or an iterator?

Arguments passed to a *@parameter are passed lazily, as an iterator tied to an array interface.

Do you have any feel for whether calling subs in P6 will have a lower overhead than P5?

That's a core design goal.

Does multi infix:max= happily combine with the multi max subs above?

Yes. That's exactly why I wrote it that way. ;-)

BTW, if anyone wants more detail, most of this is described in Apocalypse 6, summarized in Synopsis 6, and explained in the forthcoming Exegesis 6.

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