Course of Raku / Advanced / Containers / Ordered containers

Flattening and itemisation

When you put one array inside another, Raku does not automatically merge their elements. Examine the following program:

my @a = 1, 2, 3;
my @b = 4, 5;
my @c = @a, @b;

say @c.elems; # 2
say @c;       # [[1 2 3] [4 5]]

The array @c has only two elements: the arrays @a and @b. Raku keeps each container as a single item rather than spilling its contents into the outer array. The same happens for a value placed among others:

my @a = 1, 2, 3;
my @d = 0, @a, 99;
say @d; # [0 [1 2 3] 99]

When you actually want a single flat sequence, ask for it explicitly with the flat routine:

my @a = 1, 2, 3;
my @b = 4, 5;

say flat(@a, @b);       # (1 2 3 4 5)
say flat(@a, @b).elems; # 5

Itemisation (Itemization)

Sometimes you want the opposite: to protect a container from being flattened, even inside flat. The $(...) construct itemises its argument — it marks the result as a single item. Compare the previous example with this one:

my @a = 1, 2, 3;
my @b = 4, 5;

say flat($(@a), @b); # ([1 2 3] 4 5)

Here, $(@a) keeps @a as one element, while @b is still flattened into its two values. This is the $ sigil idea once more: a $ means “treat this as a single thing”.

Course navigation

← Binding with :=   |   Quiz — Nesting arrays →

💪 Or jump directly to the exercises in this section.