Indexing into a List

    sub lookup ( List \xs, Int \index ){…}

Make sure the index is valid.

    sub lookup ( List \xs, Int \index where 0 ..^ xs.elems ){…}

The `where` clause gets attached to the `Int` typecheck.

---

I think it may also apply if you have other limitations that you need to check regularly, like making sure that strings fit into a database.

    sub foo ( Str \name where .chars ≤ 256 ){…}

Which Perl6 has the ability to give that a name as if it was a type so that it can be reused.

    subset DB-Str of Str where .chars ≤ 256;

    sub foo ( DB-Str \name ){…}

    my DB-Str $name = '';

    $name = 'a' x 10000;
    # Typecheck error

---

One use of this feature is to implement the UInt type (This is almost exactly how it is implemented in the Rakudo implementation of Perl6)

    subset UInt of Int where {$_ >= 0};

---

Then again I've never read any that talk about Perl6 as already having that feature; so maybe I'm mistaken.

I would guess that they are talking about something slightly different. (That often seems to be the case)