Behaviour of feed operator pipeline - raku

Question

I'm experimenting with the feed operator using the following code:
my $limit=10000000;
my $list=(0,1...4);
sub task($_,$name) {
say "Start $name on $*THREAD";
loop ( my $i=0; $i < $limit; $i++ ){};
say "End $name";
Nil;
}
sub stage( &code, *#elements --> Seq) {
map(&code, #elements);
}
sub feeds() {
my #results;
$list
==> map ({ task($_, "stage 1"); say( now - ENTER now );$_+1})
==> map ({ task($_, "stage 2"); say( now - ENTER now );$_+1})
==> stage ({ task($_, "stage 3"); say( now - ENTER now );$_+1})
==> #results;
say #results;
}
feeds();
the task sub is just a loop to burn cpu cycles and displays the thread used
the stage sub is a wrapper around the map sub
each step of the feed pipeline maps to calling the cpu intensive task and timing it. The result of the map is the input +1
The output when run is:
Start stage 1 on Thread<1>(Initial thread)
End stage 1
0.7286811
Start stage 2 on Thread<1>(Initial thread)
End stage 2
0.59053989
Start stage 1 on Thread<1>(Initial thread)
End stage 1
0.5955893
Start stage 2 on Thread<1>(Initial thread)
End stage 2
0.59050998
Start stage 1 on Thread<1>(Initial thread)
End stage 1
0.59472201
Start stage 2 on Thread<1>(Initial thread)
End stage 2
0.5968531
Start stage 1 on Thread<1>(Initial thread)
End stage 1
0.5917188
Start stage 2 on Thread<1>(Initial thread)
End stage 2
0.587358
Start stage 1 on Thread<1>(Initial thread)
End stage 1
0.58689858
Start stage 2 on Thread<1>(Initial thread)
End stage 2
0.59177099
Start stage 3 on Thread<1>(Initial thread)
End stage 3
3.8549498
Start stage 3 on Thread<1>(Initial thread)
End stage 3
3.8560015
Start stage 3 on Thread<1>(Initial thread)
End stage 3
3.77634317
Start stage 3 on Thread<1>(Initial thread)
End stage 3
3.6754558
Start stage 3 on Thread<1>(Initial thread)
End stage 3
3.672909
[3 4 5 6 7]
The result in #result is correct (The input from $list in incremented 3 three times)
The output of the first two stages pulls/alternates but the third stage doesn't execute until the completion of all the input into stage 2
Is there an issue with my wrapper to the map sub causing this behaviour?
Also the time it takes to evaluate in the wrapper is significantly longer than a direct call to map.
Any help is appreciated.

Answer 1

The slurpy parameter is waiting to consume the entire sequence being passed to it. Consider the difference between:
perl6 -e 'sub foo($) { }; my $items := gather for 1..Inf { say $_ }; foo($items);'
perl6 -e 'sub foo(*#) { }; my $items := gather for 1..Inf { say $_ }; foo($items);'
The first example will finish, the second one will never finish. Thus you would want to change your stage function to:
sub stage( &code, $elements --> Seq) {
map(&code, $elements);
}