Course of Raku / Advanced / Control flow / given and when

Matching ranges, types, and conditions

A when block does not only compare for equality. It performs a smart match between the topic and the value you give it, so you can match against ranges, types, and conditions, not just single values.

Ranges

A range matches when the topic falls inside it. This makes given/when a natural fit for grouping numbers into bands:

my $score = 75;

given $score {
    when 90..100 { say 'A' }
    when 80..89  { say 'B' }
    when 70..79  { say 'C' }
    default      { say 'F' }
}

The value 75 falls into the range 70..79, so the program prints:

C

Types

A type matches when the topic is of that type. This lets you branch on what kind of value you have:

my $value = 'Raku';

given $value {
    when Int { say 'an integer' }
    when Str { say 'a string' }
}

Since $value holds a string, the program prints:

a string

Conditions

You can also use a comparison directly. Inside the when, the topic is available as $_, so a condition such as $_ < 0 matches when it is true:

my $n = -7;

given $n {
    when $_ < 0 { say 'negative' }
    when 0      { say 'zero' }
    default     { say 'positive' }
}

Here the first when matches, and the program prints:

negative

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