Why doesn't Perl6 check the array length after append?

Question

I've defined a new type Tuple as follows:

subset Tuple of Array where { .elems == 2 && .[0] < .[1] };
my Tuple $t = [1, 2];
say $t;  # [1 2] So far, so good.

I can't initialize it with shorter or longer array or with [2, 1], as expected. But, I can append to it:

$t.append(3);
say $t;  # [1 2 3] Ouch!

How's that possible?

Answer 1

my Tuple $t creates a $t variable such that any (re)assignment or (re)binding to it must (re)pass the Tuple type check.

= [1, 2] assigns a reference to an Array object. The Tuple type check is applied (and passes).

$t.append(3) modifies the contents of the Array object held in $t but does not reassign or rebind $t so there's no type check.

Mutating-method-call syntax -- $t.=append(3) instead of $t.append(3) -- will trigger the type check on $t.

There is specific syntax for a bounds-checked array (my @array[2] etc.) but I'm guessing that's not the point of your question.