Course of Raku / Advanced / Data types and scalars containers

Type constraints

In the Essentials chapter, it was shown on how to set the type of the variable using type constraints. It is now worth noting that it is container that is the container who starts limiting the data values that it can accept.

The following code creates a scalar container that only allows hosting integer data in it:

my Int $int;

Containers with a clearly mentioned type can only accept the values that fit in it. So the following assignments are all valid:

my Int $int;

$int = 123; # Directly an integer
say $int;

$int = 100 + 23; # The result is integer
say $int;

$int = '123'.Int; # A string converted to an integer
say $int;

Raku does not perform automatic type casting. So assigning a floating-point number (which is a Rat value in Raku) is not possible:

my Int $int;
$int = 123.45;

You get an error:

===SORRY!=== Error while compiling t.raku
Cannot assign a literal of type Rat (123.45) to a variable of type Int. You can declare the variable to be of type Real, or try to coerce the value with 123.45.Int or Int(123.45), or just write the value as 123
at t.raku:3
------> <BOL>⏏$int = 123.45;

To avoid an error, use explicit type conversion:

my Int $int;

$int = 123.45.Int;
say $int; # 123

Don’t be confused by two dots in 123.45.Int. The first dot is the decimal point, the second one is used to call the method Int on a rational (Rat) value.