Course of Raku / Advanced / Containers / Scalar containers

Type constraints

In the Essentials part, you saw how to set the type of a variable using a type constraint. It is worth stressing that it is the container that enforces this restriction on the values it is allowed to accept.

The following declaration creates a scalar container that can only host integers:

my Int $int;

A typed container accepts only the values that fit its type, so all of the following assignments are valid:

my Int $int;

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

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

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

Raku does not perform automatic type casting in this case. Assigning a floating-point number (which is a Rat value in Raku) is therefore not allowed:

my Int $int;
$int = 123.45;

You already get an error at compile time:

===SORRY!=== Error while compiling t.raku
Cannot assign a literal of type Rat (123.45) to a variable ($int) of
type Int.  You can declare the variable to be of type Real, or try to
coerce the value with 123.45.Int or Int(123.45), or just write the value
as 123.
at t.raku:2
------> <BOL>⏏$int = 123.45;

To avoid the error, convert the value explicitly:

my Int $int;

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

Don’t be confused by the two dots in 123.45.Int. The first is the decimal point; the second calls the Int method on the rational (Rat) value.

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