Course of Raku / Advanced / Containers / Scalar containers

Default values

When a new scalar variable is created without an immediate assignment, the container still holds a value — its default value. The exact default depends on the type of the variable.

The program below is not the best way to use Raku, but it shows the point:

my $int;
say $int + 5;

The output contains 5, which may be what you expected if you assumed that the default value of $int is 0. However, the program also prints a warning:

Use of uninitialized value $int of type Any in numeric context
  in block <unit> at t.raku line 2
5

To remove this uncertainty, either assign a value explicitly:

my $int = 0;
say $int + 5; # 5

Or declare a default value with the is default trait:

my $int is default(0);
say $int + 5; # 5

The default value is not limited to zero. It can be any value that you consider a good candidate, for example:

my $int is default(1);
say $int + 5; # 6

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