Perl6: Pushing an array to an array of arrays with one element seems not to work as expected

Question

Why do I get this kind of data-structure, when I push arrays to an array of arrays which has one array as its only element?

use v6;

my @d = ( [ 1 .. 3 ] );
@d.push( [ 4 .. 6 ] );
@d.push( [ 7 .. 9 ] );


for @d -> $r {
    say "$r[]";
}
# 1
# 2
# 3
# 4 5 6
# 7 8 9

say @d.perl;
# [1, 2, 3, [4, 5, 6], [7, 8, 9]]

Answer 1

This is the expected behavior described in the The single argument rule.

Perl 6 has been through a number of models relating to flattening during its evolution, before settling on a straightforward one known as the "single argument rule".

The single argument rule is best understood by considering the number of iterations that a for loop will do. The thing to iterate over is always treated as a single argument to the for loop, thus the name of the rule.

for 1, 2, 3 { }         # List of 3 things; 3 iterations
for (1, 2, 3) { }       # List of 3 things; 3 iterations
for [1, 2, 3] { }       # Array of 3 things (put in Scalars); 3 iterations
for @a, @b { }          # List of 2 things; 2 iterations
for (@a,) { }           # List of 1 thing; 1 iteration
for (@a) { }            # List of @a.elems things; @a.elems iterations
for @a { }              # List of @a.elems things; @a.elems iterations

... the list constructor (the infix:<,> operator) and the array composer (the [...] circumfix) follow the rule:

[1, 2, 3]               # Array of 3 elements
[@a, @b]                # Array of 2 elements
[@a, 1..10]             # Array of 2 elements
[@a]                    # Array with the elements of @a copied into it
[1..10]                 # Array with 10 elements
[$@a]                   # Array with 1 element (@a)
[@a,]                   # Array with 1 element (@a)
[[1]]                   # Same as [1]
[[1],]                  # Array with a single element that is [1]
[$[1]]                  # Array with a single element that is [1]

The only one of these that is likely to provide a surprise is [[1]], but it is deemed sufficiently rare that it does not warrant an exception to the very general single argument rule.

So to make this work I can write:

my @d = ( [ 1 .. 3 ], );
@d.push( [ 4 .. 6 ] );
@d.push( [ 7 .. 9 ] );

or also

my @d = ( $[ 1 .. 3 ] );
@d.push( [ 4 .. 6 ] );
@d.push( [ 7 .. 9 ] );