Types, classes & roles

Definedness & type objects

Full

Every type has an undefined "type object"; .defined tells values and types apart.

Definedness is distinct from truth. Every type has a type object — an undefined instance of the type (Int, Any, …) — and .defined distinguishes it from a real value. This is what with/without and optional parameters key on.

Values are defined, type objects are not #

my $x;
say $x.defined;
say 0.defined;
say Any.defined;

Output

False
True
False

An uninitialised $x holds Any (undefined); 0 is a defined value even though it is false; a bare type name is the undefined type object.

Nil and type objects #

say Nil.defined;
say (Int).defined;
say 42.defined;

Output

False
False
True

Int (the type object) is undefined; 42 (an instance) is defined.

Notes #

Definedness ≠ truth: 0 and "" are defined but false, which is exactly why // (defined-or) and with differ from || and if — see Logical operators.
A parameter typed Int:D requires a defined Int; Int:U requires the undefined type object — the :D/:U "definedness smileys".
Type objects are how Raku represents "no value of this type yet", replacing null with something type-aware.