Course of Raku / Advanced / Control flow

Block-related phasers

Besides the program-wide phasers, Raku has phasers tied to the life of a single block or loop.

Entering and leaving a block

The ENTER phaser runs each time execution enters a block, and the LEAVE phaser runs each time it leaves — whichever line they are written on:

say 'before block';
{
    LEAVE say 'leaving';
    ENTER say 'entering';
    say 'inside';
}
say 'after block';

The output shows that ENTER runs first and LEAVE runs last, around the body of the block:

before block
entering
inside
leaving
after block

LEAVE is especially useful because it runs even if the block is left early, which makes it a reliable place to release a resource.

A phaser can take a full { } block instead of a single statement, and you may set up several of them. When a block has more than one LEAVE, they fire in reverse order — the last one registered runs first, so the block unwinds like a stack:

say 'open A';
{
    LEAVE {
        say 'close A';
    }
    say 'open B';
    LEAVE {
        say 'close B';
    }
    say 'work';
}
say 'done';

Resource A is opened first and closed last:

open A
open B
work
close B
close A
done

This last-in, first-out ordering is exactly what you want for cleanup: whatever was set up most recently is torn down first. The program-wide END phaser behaves the same way — several END blocks also run in reverse of the order they were written.

Loop phasers

Inside loops, three more phasers mark the stages of the iteration: FIRST runs once before the first iteration, LAST runs once after the last, and NEXT runs at the end of each iteration:

for 1..3 {
    FIRST say '-- first';
    LAST  say '-- last';
    NEXT  say "-- next (was $_)";
    say "body $_";
}

The loop produces:

-- first
body 1
-- next (was 1)
body 2
-- next (was 2)
body 3
-- next (was 3)
-- last

As with the other phasers, the position of FIRST, NEXT, and LAST in the source does not matter — each runs at its own moment.

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