Course of Raku / Advanced / Containers / Contexts 🆕

Item and list context

Two further contexts decide whether something is treated as a list of several values or as a single item. This is the difference behind the @ and $ sigils.

In list context, an array spreads out into its elements. A for loop puts its argument in list context, so it iterates over each element:

my @a = 1, 2, 3;

my $count = 0;
$count++ for @a;
say $count; # 3

In item context, the same array is treated as one single value. The $( … ) contextualiser forces item context, so the loop now sees just one thing and runs once:

my @a = 1, 2, 3;

my $count = 0;
$count++ for $(@a);
say $count; # 1

This is why assigning an array to a scalar packs it as a single item rather than copying its elements: my $x = @a puts @a in item context. Note that that single element can be another container with many items in it:

my @a = 1, 2, 3;

my $var = @a;
say $var;      # [1 2 3]
say $var.WHAT; # (Array)

The $var variable now contains a single (as it’s a scalar container) object, which is an Array. You can easily prove this by treating $x as a kind of array reference (as you would call it in other languages):

say $var[1]; # 2

The matching @( … ) contextualiser does the opposite, forcing list context.

There is also sink context — the context of a statement whose value is thrown away, like a line that exists only for its side effect. When a value lands in sink context with nothing useful to do, Raku may warn about a “useless use”, which is a handy hint that you forgot to use a result.

The sigil you choose ($ versus @) is really a choice of context, and the contextualisers $( ) and @( ) let you override it where needed.

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