Subscript adverbs — :exists / :delete

Modify what a subscript does — test for a key, remove it, or ask for keys.

An adverb on a subscript changes what the access returns or does. :exists tests for a key without creating it, :delete removes it and returns its value, and :k/:v/ :kv/:p choose what a slice yields.

:exists and :delete #

my %h = a => 1, b => 2;
say %h<a>:exists;
say %h<a>:delete;
say %h;

True
1
{b => 2}

:exists reports the key is present; :delete removes a (returning its value 1), leaving just b.

Shaping a slice — :kv / :p / :k #

On a slice, these adverbs choose what comes back: :kv the keys and values flat, :p them as pairs, :k just the keys.

my %h = a => 1, b => 2, c => 3;
say %h<a b>:kv;
say (%h<a c>:p).sort;
say %h<a b>:k;

(a 1 b 2)
(a => 1 c => 3)
(a b)

Deleting from an array #

:delete works on an array element too, returning what it held and leaving a hole.

my @a = 10, 20, 30;
say @a[1]:delete;
say @a;

20
[10 (Any) 30]

Notes #

• :exists does not autovivify — testing %h<missing>:exists won't create the key, unlike a bare %h<missing> in some lvalue contexts.
• :delete works on arrays too, removing the element (leaving a hole/Nil), and returns what it removed.
• The value-shape adverbs pair with slices: %h<a b>:kv gives (a 1 b 2), :p gives pairs, :k just the keys that matched — the same family used by grep/first :k.