Course of Raku / Advanced / Control flow

Phasers

A phaser is a special block that runs automatically at a particular moment in the life of a program, rather than where it appears in the source code. You do not call a phaser; you only say when its code should run, by naming the block.

The most common phasers mark stages of the program as a whole:

• BEGIN runs as early as possible, while the program is still being compiled;
• INIT runs once at the start of the run, before the main code;
• END runs once at the very end, after the main code has finished.

The following program places these phasers among ordinary statements. Notice that the output does not follow the order of the lines in the file:

say 'main body';
BEGIN say 'BEGIN';
END say 'END';
INIT say 'INIT';

The program prints:

BEGIN
INIT
main body
END

BEGIN runs first, during compilation. Then, at run time, INIT runs before the main body. The main body prints next, and END runs last, no matter where it was written.

A common use of END is to print a summary or clean something up just before the program exits, while BEGIN is handy for work that must happen before anything else.

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