The Pearls of Raku, Issue 6: MAIN and BEGIN

Another set of tiny but useful things that can use in your coding practice in the Raku programming language.

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Default MAIN

An interesting way of providing the default values to all of the parameters of the MAIN function. Just define a multi-version that takes no arguments and calls a suitable version with the parameters you want.

multi sub MAIN() {
    MAIN('User');
}

multi sub MAIN(Str $name, Str $greeting = 'Hello') {
    say "$greeting, $name!";
}

Now, running a program without the arguments gives some meaningful result.

$ raku default-main.raku 
Hello, User!

$ raku default-main.raku John
Hello, John!

You can combine this method with regular default values of the arguments. If there is no sense to run a program with no parameters, make sure you have a descriptive USAGE example.

BEGIN phraser

The BEGIN phaser can be used to print a message before the program even starts running. This may be handy, for example, if you have a few different MAIN functions and want to have a common message for all of them.

The below (rather artificial) program prints a message even if the main code fails.

BEGIN {
    say 'This program prints the result of division by zero.';
}

multi sub MAIN() {
    say 42/0;
}

multi sub MAIN(Int $n) {
    say $n / 0;
}

Find the code of this issue on GitHub and browse for more Raku Pearls.