Course of Raku / Advanced / Containers / Introspecting containers

Using WHAT

WHAT is a pseudo-method that gives access to the type of a value. You can use it in much the same way as .^name:

my $var = 42;

say $var.^name;
say $var.WHAT;

The two print the type with a minor difference in decoration: .^name gives the bare name, while WHAT shows the type object, written in parentheses:

Int
(Int)

For a variable without a type constraint, the type starts as Any. As soon as you assign a value, both ^name and WHAT follow the type of the stored value:

my $var;
say $var.^name; # Any
say $var.WHAT;  # (Any)

$var = 'Hello';
say $var.^name; # Str
say $var.WHAT;  # (Str)

With a type constraint, the type is known immediately, even before anything is assigned:

my Str $var;
say $var.^name; # Str
say $var.WHAT;  # (Str)

$var = 'Hello';
say $var.^name; # Str
say $var.WHAT;  # (Str)

Comparing type objects

Because WHAT returns the type object itself, you can compare two of them with the value identity operator ===, which asks whether both sides are the very same value. There is only ever one type object per type, so this is a clean way to test whether two values share a type:

my $a = 42;
my $b = 100;

say $a.WHAT === $b.WHAT; # True  — both are Int
say $a.WHAT === Int;     # True
say $a.WHAT === Str;     # False

Unlike ==, which compares numbers, === compares identity, so it works for type objects (and other values) directly.

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