p6 S02: Type Objects

p6-type-objects

Type Objects

[1] Every type has an undefined prototype object associated with it. It is responsible
for creating concrete objects of that type, among other things (see S12 for more on that).


[2] When a variable is declared to be of a particular type, it is actually being assigned
this prototype object (also called the type object for short).

my Int $x;              # $x is assigned the type object "(Int)"
say $x;                 # (Int)
say $x.defined;         # False


[3] To get a concrete object, a constructor method must be called.

my Int $x = $x.new;     # $x is first assigned the type object, which is then used
                        # to create and assign a concrete Int object to $x.
say $x;                 # 0
say $x.defined;         # True

TODO: Constructor with arguments.


[4] The type name used as a value is the type object itself.

Thus, the following statements are equivalent. All assign the type object to $x.

my Int $x;
my Int $x = Int;
my $x     = Int;

And so are the following. All create a concrete Int object and assign to $x.

my Int $x = $x.new;     # Shorthand: my Int $x .= new;
my Int $x = Int.new;
my $x     = Int.new;


[5] The type object responds to a function call interface. Any argument passed to the
call is coerced to the type associated with the type object.

my $x = Int("5");       # String argument coerced into Int.
say $x;                 # 5

my $x = Int(5);         # Number argument coerced into Int.
say $x;                 # 5

my $x = Int();          # Void argument coerced into Int.
say $x;                 # 0

Equivalent longer forms.

my Int $x = $x("5");
my Int $x = $x(5);
my Int $x = $x();