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do
Combined from primary sources listed below.

In Control flow§

See primary documentation in context for do.

The simplest way to run a block where it cannot be a stand-alone statement is by writing do before it:

# This dies half of the time
do { say "Heads I win, tails I die."; Bool.pick } or die; say "I win.";

Note that you need a space between the do and the block.

The whole do {...} evaluates to the final value of the block. The block will be run when that value is needed in order to evaluate the rest of the expression. So:

False and do { 42.say };

...will not say 42. However, the block is only evaluated once each time the expression it is contained in is evaluated:

# This says "(..1 ..2 ..3)" not "(..1 ...2 ....3)"
my $f = "."; say do { $f ~= "." } X~ 1, 2, 3;

In other words, it follows the same reification rules as everything else.

Technically, do is a loop which runs exactly one iteration.

A do may also be used on a bare statement (without curly braces) but this is mainly just useful for avoiding the syntactical need to parenthesize a statement if it is the last thing in an expression:

3, do if 1 { 2 }  ; # OUTPUT: «(3, 2)␤»
3,   (if 1 { 2 }) ; # OUTPUT: «(3, 2)␤»

3,    if 1 { 2 }  ; # Syntax error

As a consequence, do does not run blocks that, by their syntax, must be functions. For example, if -> is used to specify a signature, do treats these as single-expression statements.

Thus, adding -> to our first example prevents the closure from being evaluated:

# This never dies and never prints "Heads I win, tails I die."
do -> { say "Heads I win, tails I die."; Bool.pick } or die; say "I win.";