Re: Perl6 Contest #2: P6 That Doesn't Look Like P5

[blokhead]

Here's what I believe to be a "correct" implementation although it doesn't work with my copy of pugs.

It's a slightly functional way of doing it. (Assuming my understanding of lexically-scoped subs is correct) I set up a recursive "incrementer" function which increments one position of @pos and "carries over" via a recursive call. It's tail-recursive, so if the compiler is smart, we don't lose a lot by making it recursive instead of iterative.

sub NestedLoops (*@loop) returns Ref {
    my @pos = 0 xx (@loop.elems - 1), -1;

    my sub incr($i) {
        if ( ++@pos[$i] == @loop[$i].elems ) {
            @pos[$i] = 0;
            return $i ?? incr($i - 1) :: 0;
        }
        return 1;
    };

    return sub {
       incr(@loop.end) or return;
       zip(@loop, @pos) ==> map -> $a, $i { $a[$i] };
    };
};

my $iter = NestedLoops( [0..2], [0..2] );
my @group;

while ( @group = $iter() ) {
     say ~@group;
}

From what I can tell, there are two things currently holding back pugs:

• Doesn't appear to be support for the ==> operator yet. But I wasn't able to get zip().map to work correctly either.
• map -> $a, $i doesn't take two at a time.

You can fix that by just substituting Limbic~Region's map statement for the line containing zip. I was hoping there would be an even easier way to write this line. Something like:

  @loops >>.[]<< @pos

But pugs didn't like that..

[kelan]

You can always simulate your own n-ary map using gather:

gather {
  for @loop Y @pos -> $arr, $i {
    take $a[ $i ];
  }
}

Although allowing map to take multiple items from the list at once would be nicer, and it's something I've wanted rather often.

[iblech]

Although allowing map to take multiple items from the list at once would be nicer, and it's something I've wanted rather often.

Sure, works in current Pugs! :)

  @result = map { $^a + $^b } 1,2,3,4;
  # 3,7

  @result = map -> $a, $b { $a + $^b } 1,2,3,4;
  # 3,7

  # And:
  $result = reduce { $^a + $^b * $^c } 1,2,3,4,5;
  # same as
  $result = (1 + 2 * 3) + 4 * 5;
  # i.e.
  # 27

--Ingo

[kelan]

If you have your trusty zipwith available:

sub zipwith ( Code $with, *@lists ) {
  gather {
    while any( @lists ) {
      take $with.( @lists>>.shift );
    }
  }
}

then you can change that zip line to:

zipwith( { $^a[ $^i ] }, @loop, @pos );