Course of Raku / Advanced / Control flow / Statement prefixes 🆕

do and the value of a block

A block on its own is a statement, not an expression, so you cannot normally assign it to a variable. The do prefix turns a block into an expression whose value is the value of its last statement:

my $x = do {
    my $a = 3;
    $a + 4;
};

say $x; # 7

Inside the block you can declare variables, run several statements, and compute a result; do hands back whatever the block evaluates to. This is handy when producing a value needs more than a single expression.

do also works in front of the control-flow statements, letting them return a value too:

my $sign = do given 5 {
    when * > 0 { 'positive' }
    when * < 0 { 'negative' }
    default    { 'zero' }
};

say $sign; # positive

Here do given turns the whole given/when into an expression that yields positive. The same works with do if and do for. In short, do is how you use a block, or a control structure, in a place that expects a value.

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